Typical star-forming galaxies at these early epochs are clumpy, rotating disks with high star formation rates, high molecular gas fractions and highly turbulent interstellar media. Furthermore, recent studies argue that these objects have less dark matter than expected from nearby spirals and cosmological simulations in the Lambda-CDM framework. To understand how galaxies evolve to the well-ordered morphologies observed locally, spatially-resolved studies are essential. In particular, Integral Field Unit (IFU) observations can provide the full kinematic information required to study the mass distribution of galaxies. In this talk I will present our recent efforts with the KMOS integral-field spectrograph at VLT to characterise the dynamics of star-forming galaxies at cosmic noon, focusing in particular on the relation between baryonic and non-baryonic components in galaxies.
Friday 17 November 2023
Title : The impact of rotation and turbulence on core-collapse supernovae
and in a shallow water analogue experiment
Speaker : Thierry Foglizzo (CEA Saclay)
Abstract :
The development of hydrodynamical instabilities during the first second after core bounce is a key ingredient in the explosion mechanism of massive stars.
It affects the birth properties of neutron stars and black holes and generates specific signatures in gravitational waves and neutrinos.
The advective-acoustic mechanism of the standing accretion shock instability (SASI) is well established in a radial collapse but some properties of its spiral model in a rotating stellar core challenge our understanding since Walk+23.
Guided by experimental results in a shallow water analogue, we use a perturbative study in the adiabatic approximation to reach an analytical understanding of the instability mechanism.
Analytical estimates of SASI growth rate and oscillation frequency are obtained and the destabilizing effect of rotation is clarified.
By calculating the effects of viscosity and thermal diffusivity, we further clarify the relative roles of vorticity and entropy perturbations in the advective-acoustic mechanism.
This calculation also reveals the sensitivity of SASI to pre-collapse turbulence, and demonstrates the importance of a fine radial resolution to correctly account for SASI in numerical simulations.
Friday 10 November 2023
Title : Towards a comprehensive understanding of the origin of globular clusters and of their multiple stellar populations
Speaker : Francesco Calura (Osservatorio Astronomico di Bologna)
Abstract :
A clear understanding of the physical mechanisms behind the origin of globular clusters (GCs) and the origin of their multiple populations (MPs) is still lacking. Moreover, thanks to the first observational studies of their high-redshift progenitors, GCs have regained significant interest also from a cosmological perspective. Achieving a comprehensive theoretical understanding of GCs is now required to interpret the wealth of data that is already at hand and to guide future observations.
To this aim, in this project, we present a series of 3D hydrodynamic simulations to probe with unprecedented detail the earliest phases of GCs evolution, starting from the collapse of dense clumps at very high redshift.
Then, we focus on isolated systems and study the role of several fundamental physical ingredients, such as thermal and radiation stellar feedback and cluster rotation, in shaping the main properties of MPs.
Finally, we study the long-term dynamical evolution of a massive GC in the Milky Way through a series of dedicated N-body simulations, in which we investigate the role of mass loss and the main structural parameters that regulate it.
Thursday 19 October 2023 — PhD defence
Title : Sédimentation de grains poreux dans les disques protoplanétaires : influence sur l’épaisseur de la couche de poussière et la formation de planétésimaux
Speaker : Stephane Michoulier (CRAL)
Abstract :
Dans le paradigme de la formation planétaire, le processus par lequel les agrégats de
poussière sous-micrométrique à millimétriques croissent dans les disques protoplanétaires pour former des planétésimaux demeure encore largement méconnu. La coagula0on des grains de poussière se heurte essen0ellement à deux obstacles majeurs : la barrière de la dérive radiale et la barrière de la fragmenta0on, qui empêchent la survie des agrégats et, par conséquent, la forma0on de planètes. Une solu0on poten0elle pour surmonter ces obstacles est de prendre en compte la porosité des grains, ce qui permet à la poussière de croître plus rapidement et plus longtemps tout en étant moins sensible à la fragmenta0on que les grains compacts (Garcia 2018).
Un modèle d’évolu0on de la porosité pouvant être u0lisé dans des simula0ons 3D a été ini0alement développé par Anthony Garcia (Garcia 2018, Garcia & Gonzalez 2020). J’ai implémenté ce modèle dans le code d’hydrodynamique SPH Phantom et y ai ajouté de nouveaux effets, principalement destruc0fs, tels que la rupture rota0onnelle, le rebond, l’érosion, et la compac0on lors de la fragmenta0on. J’ai écrit un second code, 1D, Pamdeas qui inclut aussi l’ensemble des processus physiques, offrant ainsi une compréhension plus facile de l’évolu0on des grains de poussière en fonc0on des phénomènes choisis. J’ai également implémenté un nouveau schéma implicite SPH pour calculer la fric0on entre le gaz et la poussière.
Au fil de ce manuscrit, je présenterai les nouveaux modèles permeXant de prendre en compte un plus large éventail de phénomènes physiques liés à la poussière et à son évolu0on. Je montrerai les résultats des simula0ons de disques protoplanétaires avec des grains poreux cons0tués de silicates ou de glace d’eau, tout en discutant de l’efficacité de leur croissance par rapport à celle des grains compacts, en confirmant les résultats antérieurs. J’insisterai également sur l’impact des différents phénomènes physiques inclus dans ma thèse. Je trouve que la compac0on lors de la fragmenta0on est l’effet le plus significa0f, permeXant une compac0on des grains avec des facteurs de remplissage cohérents par rapport aux récents résultats observa0onnels, sans limiter de manière excessive la taille des agrégats poreux. En revanche, des phénomènes tels que le rebond ou la rupture par rota0on ont beaucoup moins d’effet sur l’évolu0on des grains de poussière.
Ces résultats contribueront à une meilleure compréhension des mécanismes de coagula0on de la poussière et ouvriront de nouvelles perspec0ves dans l’interpréta0on des observa0ons de la poussière au sein des disques protoplanétaires.
Friday 13 October 2023
Title : Resolving star formation processes at 10-100s pc resolution in distant galaxies with JWST/NIRCam
Speaker : Adélaïde Claeyssens (Stockholm University)
Abstract :
Distant galaxies present a very irregular morphology dominated by compact sub-structures called “clumps”. The physical properties of these star-forming systems remain relatively unexplored and their role in galaxy formation and evolution is not clear. The first detections of UV-bright clumps from z=10 to z=1 indicate that these star-forming clumps could be a major mode of star formation and galaxy assembly.
However, resolving structures at 10-100s pc scales in high redshift galaxies (z>1) is hardly achievable with current telescopes even with space-based observations. Combining the unprecedented sensitivity and spatial resolution of JWST with the natural gravitational lens telescopes is the only way to reach hundred/sub-hundred pc resolutions in hundreds of galaxies, necessary to resolve individual star-forming systems and star clusters at UV-optical wavelengths. I will present the first results on resolved high redshift star formation processes obtained from JWST/NIRCam observations of strongly lensed galaxies observed in multiple galaxy cluster fields (SMACS0723, WHL0137, Abell2744). The optical restframe, probed with the JWST, enables us to measure physical properties of >2000 clumps (age, mass, extinction). We derive effective radii from <10 to 100s pc and masses ranging from 10^5 to 10^9 Msun, overlapping with massive star clusters in the local universe. Comparing these results with the most recent hydrodynamical simulations of galaxies, we can understand the physical processes involved in the formation and evolution of these clumps (such as gas turbulence, stellar feedback, galaxy mergers and bulge formation). This study show the potential of JWST observations for understanding the conditions under which galaxies assembly and evolve across the cosmic time and allow us to study for the first time the formation and the role of star clusters in rapidly evolving galaxies.
Friday 6 October 2023
Title : GATOS and EIFIS : Two new instrumentation concepts for the 10.4m GTC
Speaker : Antonio de Ugarte Postigo (Observatoire de Cote d’Azur)
Abstract :
In this talk I will be reviewing the instrumentation suite of the 10.4m GTC telescope and present two new instrumentation concepts in which I am involved and that are being proposed as future instruments :
GATOS is an 8-band imager and spectrograph with high time resolution capabilities similar to SCORPIO, which we presented to Gemini and is currently being built for the Gemini South telescope. GATOS is able to perform imaging in 8 simultaneous optical and NIR channels (g, r, i, z, Y, J, H, Ks) with a field of view of 3’x3’ or performing long-slit spectroscopy simultaneously covering the spectral range between 3700 and 23500 Å and a resolving power of 4000. These observation modes can be operated in high time-resolution thanks to the use of frame-transfer detectors in the optical and state of the art NIR detectors. The design uses a common cryogenic focal plane for all bands and then divides the light into different channels by the use of high-efficiency dichroics. The focal plane allows for the addition of a integral field spectroscopy unit and/or a spectropolarimeter.
EIFIS is a conceptual study of a wide field integral field spectrograph, which uses a modular design to explore the widest possible achievable field of view with a seeing-limited spatial resolution an a resolving power of 2400. The design uses double spectrographs to cover the complete optical spectral range and large format detectors with double pseudo-slits to minimise the number of individual spectrographs required.
Friday 29 September 2023
Title : Turbulence in outer protoplanetary disks and debris disks
Speaker : Can Cui (University of Toronto)
Abstract :
Turbulence is essential to many fundamental processes in protoplanetary disks, including angular momentum transport, dust evolution, and planet migration. I will focus on two instabilities that can drive turbulent motions in outer disks. In the first part of this talk, a series of global 3D non-ideal MHD simulations via Athena++ code will be presented. The outer disk is found to be weakly MRI turbulent, and annular substructures arise due to magnetic flux concentration. The weak MRI turbulence permits the growth of the vertical shear instability. In the second part of this talk, the saturation of the vertical shear instability mediated via a parametric instability will be covered. Once the parametric instability prevails, it is anticipated that the vertical shear instability is far more incoherent than extant published simulations suggest. Finally, I will touch on a new topic I recently work on - debris disk gas evolution.
Friday 15 September 2023
Title : Shaping the Future with Freeform Optics : An Emphasis on Design
Speaker : Jannick P. Rolland (University of Rochester)
Abstract :
Freeform Optics enables the design of more compact or more complex optical systems across a wide range of applications. In this talk, we will demystify designing with freeform surfaces, yet demonstrate their power in optical design. Designing for manufacture is important for affordability, and this is even more critical with freeform optics due to their complexity. We will discuss aspects of design for manufacture and briefly introduce a novel optical component, the metaform.
Short bio :
Jannick P. Rolland is the Brian J. Thompson Professor of Optical Engineering at the University of Rochester and the Chief Technology Officer of LighTopTech Corp., a startup she co-founded in 2013 in biotechnology. At the University of Rochester, she is the director of the NSF I/UCRC Center for Freeform Optics (CeFO), and the director of the R.E. Hopkins Center for Optical Design and Engineering. She earned an optical engineering diploma from the Institut D’Optique Théorique et Appliquée, France, and MS and PhD degrees in Optical Science from the JWC College of Optical Sciences at the University of Arizona. Professor Rolland is a Fellow of Optica, SPIE, and NAI and an inaugural member of the IEEE VGTC Virtual Reality Academy. She is the recipient of the 2014 David Richardson Medal, and the 2020 Joseph Fraunhofer Medal /Robert M. Burley Prize.
Friday 8 September 2023
Title : Cryogenic AIT tools for HARMONI
Speaker : Adrien Girardot (CRAL — internal seminar)
Abstract :
HARMONI (High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph) is an integral field spectrograph designed to operate within the challenging environment of a 10^-5 mbar pressure and at a temperature of 130K. To overpass these challenges, Assembly Integration and Testing (AIT) tools have been meticulously developed. In this internal seminar, I will present 2 of these tools : the PIC (Photogrammetric Inside Cryostat) and the Chief Ray Tool. This will be an opportunity to present my 3 years of work as an apprentice at CRAL.
Friday 23 June 2023
Title : Galactic Multi-scale Simulations of GMC and Star Cluster Formation
Speaker : Ralph Pudritz (McMaster University)
Abstract :
The advent of ALMA, JWST, and a host of recent high resolution atomic hydrogen, molecular gas, and dust surveys are transforming our understanding of star formation over a very wide range of physical scales in galaxies. From large scale kpc filaments and superbubble structures, to giant molecular clouds (GMCs) on 100 parsec (pc) scales, star clusters (1 pc) and the sub pc scale filaments in which we observe individual stars form, a new hierarchical and dynamic picture of star formation is emerging. I will review recent progress in the observations, simulations, and theory of star formation that addresses the physical connections between structure formation from the galactic to pc scales. I will then discuss our own recent galactic multi-scale, zoom-in simulations that allows us to track the formation of structure from galactic to sub pc scales in a magnetized, Milky Way like galaxy undergoing supernova driven feedback processes.
A wide variety of structures including superbubbles, kpc atomic gas filaments, smaller scale filamentary GMCs, and star cluster regions form. Gravitational instability of filaments on these different scales and filamentary flows within them, drive GMC and cluster formation respectively. In quieter regions, galactic shear can produce filamentary GMCs within flattened, rotating disk-like structures on 100 pc scales. Strikingly, magnetic field topologies associated with such disk structures are highly helical. This variety seems to reflect recent observational 3D structure maps of nearby GMCs.
Friday 16 June 2023
Title : What is the origin of the different kinematic morphologies of early-type galaxies ?
Speaker : Michal Bilek (Paris Observatory LERMA)
Abstract :
Early-type galaxies (i.e. elliptical and lenticular) are divided into slow and fast rotators according to the appearance of their maps of line-of-sight velocity. Fast rotators show clear ordered rotation, while slow are supported mostly by velocity dispersion.
I will speak about our work on investigation of the origin of this diversity. Inspired by cosmological simulations, we assumed that galaxies first form as fast rotators and then mergers transform some of them to slow rotators. We investigated the correlations of a measure of rotational support with various properties of galaxies that are sensitive to mergers. These include stellar ages, the presence of tidal features, and kinematically distinct cores. Each of these parameters is sensitive to a different type of merger and has a different lifetime.
The found correlations, or their lack, together with observations of high-redshift universe, are explained the easiest, if the rotation support of early-type galaxies was decreased by multiple minor wet mergers more than 10 Gyr ago.
Friday 9 June 2023
Title : How (not) to make stars and planets
Speaker : Daniel Price (Monash)
Abstract :
What have we learnt about the formation of stars and planets by imaging young stars, and in particular the swirling discs of material around them known as protoplanetary discs ? Can you grow a star by accreting from a disc ? How quickly does the material in the disc get converted into planets ? Can we understand what causes the outburst events on young stars ? Can we detect young planets directly ? How does this relate to the formation of our own solar system ? I will try to answer all of these questions, and more, in just 45 minutes of your life.
Tuesday 30 May 2023
Title : Opportunities for Imaging the Planet Forming Environment with ALMA
Speaker : Ian Czekala (Penn State)
Abstract :
In recent years, high angular-resolution sub-mm wave dust continuum observations of protoplanetary disks have helped discover radial substructures and azimuthal asymmetries, yielding new insights into the planet-forming environment. With its capability to spectrally resolve molecular line emission, the Atacama Large Millimeter Array (ALMA) provides an even more informative dimension with which to study protoplanetary structures, chemical inventories, and dynamics. In the first part of this talk, we will describe how we use the bulk Keplerian motion of protoplanetary disks to precisely (<5%) constrain the central stellar mass, and, when combined with stellar orbits, how these constraints can unlock the architectures of multiple-star protoplanetary systems. We will discuss our survey of 20 circumbinary systems in the context of the dozen Kepler circumbinary (or "Tatooine") planets, its implications for a coplanar planet/disk population around short-period binaries (periods < 40 days), and the possibility of a set of widely dispersed orbits around wider binary systems (periods > months). In the second part of this talk, I will highlight the results from the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program ("The Chemistry of Planet Formation") and discuss ongoing efforts to use molecular emission to search for (proto)-planetary kinematic signatures in the exoALMA Large Program. Central to these efforts are robust, non-parametric imaging methodologies, broadly categorized under the umbrella of "Regularized Maximum Likelihood" (RML) techniques. We will conclude with preliminary results from our open-source "Million Points of Light" RML imaging framework and discuss potential high-fidelity imaging workflows for ALMA spectral line datasets.
Friday 12 May 2023
Title : How to weigh your protoplanetary disc (without using a scale !)
Speaker : Benedetta Veronesi (CRAL — internal seminar)
Abstract :
Young circumstellar discs are made of almost invisible molecular hydrogen, making it extremely challenging to weigh it. Yet, the local mass determines the amount of material available for assembling planets and controls solid evolution by setting the strength of dust/gas coupling. When the disc-to-star mass ratio becomes of order roughly 0.1, the gravity of the disc may trigger gravitational instabilities that generates spirals inside the disc. Thus, knowing the protoplanetary disc mass is of paramount importance to better understand the evolution of these systems. New methods have been developed, but the usual disc weighting by CO-isotopologues or dust remain uncertain, since both these tracers rely on conversion factors and on the assumption that the disc is locally optically thin.
We propose and benchmark a novel method for weighting massive discs. When high resolution kinematic data are available, we extract the velocity curve and fit it with an analytical model where both the disc and the star contributions are taken into account to obtain the disc mass (Veronesi et al. 2021, Lodato et al. 2023). We test and demonstrate the usefulness of the method by performing several hydrodynamic and radiative transfer simulations of self-gravitating protoplanetary discs with PHANTOM (SPH - Price et al. 2018) and MCFOST (Pinte et al. 2009), exploring a range of disc masses.
Friday 5 May 2023
Title : Mapping the Circumgalactic Medium and Intergalactic Medium through Lya and MgII Emission
Speaker : Yucheng Guo (CRAL — internal seminar)
Abstract :
Based on the extremely deep MUSE observation on the Hubble Ultra Deep Field, we map the emission from the intergalactic medium and the circumgalactic medium at a physical scale from hundreds of kpc to around 10 kpc. Over this large range of distance, we observe different physical processes occurring at different distances to the galaxies. In this talk I will report three main results : (1) We measure the extended Lya emission out to approximately 270 kpc at 3 < z < 4. (2) From the galaxy center to approximately 70 kpc around the galaxy, we observe an average blueshift of the Lya line, strongly suggesting the presence of large-scale gas inflows. (3) Within 15 kpc, we observe the bi-polar galactic outflows traced by anisotropic MgII emission around galaxies at z around 1. We demonstrate that this cool and metal-enriched outflow is prevalent among the massive galaxy population.
Friday 28 April 2023
Title : Spectrograph design : some challenges, strategies and technologies
Speaker : Alexandre Jeanneau (CRAL — internal seminar)
Abstract :
Although the basic layout of optical spectrographs has remained largely unchanged over recent decades, their design and manufacturing have become more challenging as their performance targets have become more ambitious. In this talk, I will highlight some challenges, strategies and technologies associated with the design of next generation, moderate resolution spectrographs such as those at the heart of BlueMUSE, MSE and WST.
Friday 7 April 2023
Title : Intermediate-mass black holes in dwarf galaxies - a missing link in the understanding of early black hole formation and evolution
Speaker : Dieu Nguyen Duc (CRAL — internal seminar)
Abstract :
The fraction of low-mass galaxies hosting an intermediate-mass black hole is sensitive to how the black hole seeds formed in the early universe. While ones know that all massive galaxies contain a supermassive black hole at their center, the low-mass end of the black hole occupation fraction and the black hole mass and host galaxies’ macroscopic properties correlations, i.e., what happens "under the peaks" in the mass spectrum, are observationally unconstrained. This quantity is crucial for distinguishing between black hole seed formation theories and predicting detection rates from next-generation gravitational-wave observatories. My work is progressing on measuring the low-mass end of the black hole occupation fraction and determining the behaviors of the black hole-galaxy scaling relations to provide constraints for black hole seed formation theories. In this talk, I will summarize my past work in dynamic efforts on these measurements using high-quality observation from HST, VLT/SINFONI, Gemini/NIFS, and ALMA. In the second part of the talk, I will present the simulated attempts with ELT/HARMONI observations which will revolutionize our understanding of the black hole seeding formation mechanisms thanks to their unprecedented spatial/spectral resolution. Those will ultimately advance our understanding of the physics of the lightest central black holes in dwarf galaxies.
Friday 31 March 2023
Title : Structural evolution of galaxies since cosmic noon
Speaker : Stijn Wuyts (University of Bath)
Abstract :
In this talk I will discuss the geometry, structure and morphological transformation of star-forming and quiescent galaxies from the peak of cosmic star formation to the present day. Resolved observations from multiple wavelengths and tracers enable us to reconstruct where stars form within galaxies and how stellar distributions are assembled, while simultaneously revealing insights into the evolving 3D spatial distributions of stars and dust. Integral-field spectroscopy adds to this the kinematic perspective, probing both gravitational and non-gravitational motions, and with it signatures of dark matter content and feedback in action. I will draw from HST, JWST, ALMA and IFU observations to illustrate the structural make-up of galaxies across cosmic time, and will draw connections to state-of-the-art cosmological simulations.
Friday 24 March 2023
Title : Toward Unsupervised Predictive Adaptive Optics Control
Speaker : Éric Thiébaut (CRAL — internal seminar)
Abstract :
In the last years the AiRi team has been involved in several advances in adaptive optics (AO) systems. We have built the AO system of the Themis solar telescope, developed its real-time control from scratch, and closed the loop in December 2020. We are part of the SAXO+ study for the new AO system of SPHERE. We carry out innovative R&D for deformable mirrors, wavefront sensors, data post-processing, and real time control of AO systems to achieve better correction by the AO system, less diffused light, and higher contrasts for the current and future largest telescopes. All these are needed to detect and study faint objects or structures in the close environment of stars : exoplanets, proto-planetary disks, etc. In the framework of the PEPR Origins, we will lead 3 work-packages which are as many opportunities to pursue this R&D for AO systems. In this presentation, I will quickly summarize the objectives of the work-package 5.3 of the PEPR Origins : "Unsupervised Predictive Adaptive Optics Control" before focusing on recent related results in the team. I will present our Toolkit for Adaptive Optics systems (TAO) used for the real-time control (RTC) software of the Themis AO system and of the prototype AO system at the National Astronomical Research Institute of Thailand (NARIT). TAO is a lightweight flexible platform designed to implement other algorithms than the ones used in current AO systems. I will give an overview of new signal processing algorithms that have been (or will be) implemented in TAO : real-time image pre-processing with estimation of the noise statistics, self-calibrated solar wavefront sensing, model-based control loop, and new tools to model the statistics of the turbulence.
Friday 17 March 2023
Title : An Emerging Hybrid Technology Towards Ultra-light & Self-correcting, "Live" and "Fast" Deformable Mirrors
Speaker : Gil Moretto (CRAL — internal seminar)
Abstract :
Scenario 1 — Astronomical imaging of ever fainter objects as well as imaging the Earth from space require much higher angular resolution and dynamic range than current optical telescopes can deliver. Mirrors, the key elements of these systems, are technologically difficult to improve because they must maintain an exceedingly precise shape while resisting deformation from gravity and variable wind loads in the open environments in which they operate. For over a century, the primary technology for shaping mirrors has been that of “abrasive polishing,” with stiffness to resist environmental deformation provided by steel and glass which increase the mass significantly. This is impractical for systems larger than a few meters – the 10m Keck telescopes exceed this limit and depend on cumbersome active mirror surface shape control. Our funded EIC (European Innovation Council) project will establish a new paradigm : we will shape thin, very smooth, “fire-polished,” lightweight glass to a predetermined curvature and generate dynamically controlled stiffness by using the addressable energy of electroactive polymers to resist environmental deformation – making what we call a “Live-Mirror.”
Scenario 2 — A significant progress in the field of optical telecommunications, astronomy and microscopy were achieved these last decades. Thanks to the control of optical aberrations using active shape mirror technology. Yet, commercial mirror technologies are still suffering from fragility, temporal bandwidth, dynamical range, and size limitations and their cost. In the frame of France 2030 PEPR Origins our FlexSiMirror project aim to develop a new lightweight, hybrid and high-speed mirror technology based on the state processes to fabricate defects-free and self-supporting silicon membrane with locally doped areas and also develop a disruptive technology not using standard ceramics but rather 3D printed electroactive polymer actuators to produce, at higher temporal bandwidth (1-to-3kHz), large and very accurate displacements compliant with the needs of future instruments seeking for aberrations corrections & control.
Friday 10 March 2023
Title : The PEPR Origins and the extreme adaptive optics optimal wavefront measurement and control project
Speaker : Maud Langlois (CRAL — internal seminar)
Abstract :
Understanding the origin of life, discovering and characterizing other Earths are fundamental scientific objectives, which are booming on the international scene. The effectiveness of this quest for Origins is deeply dependent on the technological advances initiated by researchers in the field. The PEPR Origins focuses on removing precisely identified technological barriers to enable decisive progress along five lines of research of national excellence : (i) the detection and characterization of exoplanets by direct imaging, (ii) the analysis of extraterrestrial samples, with or without biological risks, (iii) the study of the Earth as a whole and as a habitable planet, (iv) numerical modeling and data analysis and (v) laboratory experimentation in astrobiology and bio-analysis of samples from ancient Earth or Mars. After presenting in detail the overall project, which is organized in 18 focused projects, identified to break the technology locks, I will focus the extreme adaptive optics optimal wavefront measurement and control project.
Tackling exoplanet science requires such an ambitious R&D program supporting the development of high contrast imaging solutions that beat down the photon noise of a bright host star. One strategic opportunity is the ELT/PCS instrument for which the French community is actively preparing : on its critical path lies the need for an integrated new AO system supported by modern wavefront sensor concepts. Using a unique expertise developed at CRAL we propose to develop a fast and extreme precision wavefront sensor prototype optimized for telluric planet detection with PCS/ELT, with prior experimentations on smaller diameter telescopes. Deployed on infrastructures of increasing angular resolution : the VLT, and the ELT, these developments will give us the means to image and characterize planets similar in size and mass to our Earth.
Friday 3 March 2023
Title : The dynamics, star formation and chemical properties of high redshift galaxies as seen with KMOS and MUSE
Speaker : Mark Swinbank (Durham University)
Abstract :
The epoch around redshift 1–2 appears to be the era where galaxies transition from clumpy, irregular morphologies to smoother, disk-like systems and the Hubble Sequence starts to emerge. The last decade has seen a major advance in the study of the internal properties of galaxies at this epoch. In this talk, I will discuss recent results, in particular from KMOS (and MUSE !) that study the internal dynamics, chemistry, star formation and dark matter properties in large samples of high redshift galaxies.
Wednesday 22 February 2023, 2 pm
Title : JWST observations of ALMA [OIII] 88 μm emitters in the epoch of reionization
Speaker : Takuya Hashimoto (University of Tsukuba)
Abstract :
Understanding properties of galaxies in the epoch of reionization (EoR) is a frontier in the modern astronomy. With ALMA, it has become possible to detect far-infrared fine structure lines (e.g., [CII] 158 μm and [OIII] 88 μm) and dust continuum emission in star-forming galaxies in the EoR. In this talk, we will first show ALMA observations that have demonstrated that i) some [OIII] 88 μm emitters have matured stellar populations at z > 6, implying early star formation activity at z > 10, and that ii) high-z star-forming galaxies typically have very high [OIII] 88 μm-to-[CII] 158 μm luminosity ratios ranging from 3 to 12 or higher, indicating interstellar media of high-z galaxies could be highly ionized. Secondly, we will discuss initial results of a medium-sized JWST GO1 program that targets a sample of 13 z = 6-8 ALMA [OIII] 88 μm emitters with NIRCam and NIRSPec IFU modes (PIs : J. Álvarez-Márquez & T. Hashimoto). Our JWST GO1 program, in conjunction with ALMA data, will characterize the stellar, nebular, and dust properties of these [OIII] 88 μm emitters and place this galaxy population in a context of the cosmic reionization.
Friday 3 Feb 2023
Title : Experimental mechanics, some approaches and their tools
Speaker : Matthieu Guibert (CRAL — internal seminar)
Abstract :
Experimental mechanics has been used for a long time in dedicated research labs, from strain and stress estimations on static structures, to vibrations measurements on complex systems. The increasing use of numerical tools, and the availability of large volume of data through image acquisitions has enhanced new techniques to measure these parameters. Finally, the cost reduction observed on the last 4 decades on both computers and numerical cameras has now made these tools standard in industrial and research applications. In this talk, I’ll present some of these techniques and their available tools. I will also present their use in reverse modelling of structure, and show some example of applications done in CRAL or in my previous professional experiences.
Friday 27 January 2023
Title : X-ray emissions from the large scale structure of the Universe
Speaker : Johan Comparat (MPE)
Abstract :
In this presentation I will discuss recent (eROSITA based) observational advances of the large scale structure in the X-ray wavelength range. First I will focus on how X-ray selected AGN populate the cosmic web. Then I’ll discuss observations of the circum-galactic medium and their implications for the field of galaxy formation and evolution.
[Relevant articles, arxiv 2201.05169, 2207.09242, 2301.01388]
Friday 20 January 2023
Title : Implications of AGN feedback on high redshift galaxies
Speaker : Flora Stanley (IRAM)
Abstract :
One of the main open questions of galaxy evolution is how active galactic nuclei (AGN) can affect the ongoing star-formation and interstellar medium (ISM) properties of galaxies. To reproduce the properties of galaxies in the local universe, cosmological and semi-analytical models of galaxy formation and evolution find it necessary to use AGN feedback as a regulator of star formation, in the form of heating and outflows of the surrounding gas. To find observational evidence that AGN do have a significant role in regulating galaxy growth, we need to study the star formation and ISM properties of active galaxies. In this seminar I will present the main results from my research on the SFR and ISM properties of AGN across cosmic time.
Wednesday 18 January 2023, 2 pm
Title : Formation of black holes in the PI mass gap
Speaker : Guglielmo Costa (CRAL — internal seminar)
Abstract :
In recent years, detailed theoretical models of the final evolutionary stages of very massive stars have shown that pair-instability (PI) is expected to open a gap in the mass spectrum of black holes (BHs) between about 40 – 65 and 120 solar masses (Msun). This result is in tension with the detected binary BH merger GW190521 by the LIGO-Virgo collaboration, with a primary BH mass of 85 Msun and a secondary BH mass of 66 Msun. In this talk, I present our recent work about the formation of BHs in the PI mass gap via star – star collisions in young stellar clusters. To avoid PI, the stellar-collision product must have a relatively small core and a massive envelope. We investigated this issue by means of hydrodynamical simulations with the smoothed particle hydrodynamics (SPH) code STARSMASHER and detailed stellar evolutionary models with PARSEC and MESA codes. In this work we simulated and analysed the collision between a core helium burning star of about 58 Msun and a main-sequence star of about 42 Msun. Finally, we find that the post-collision star evolves through all the stellar burning phases until core collapse, avoiding PI. The final BH mass is about 87 Msun. Therefore, we confirm that the collision scenario is a suitable formation channel to populate the PI mass gap.
Friday 13 January 2023
Title : Galaxies as tracers and agents of cosmic reionization in the JWST era
Speaker : Jorryt Matthee (ETH Zürich)
Abstract :
Reionization marks the last major phase transition of matter in the Universe and its completion had crucial impact on the formation of the smallest galaxies. While reionization roughly encapsulated the first Gyr of cosmic time, the precise timing, topology and the sources of ionizing photons are unknown. Did reionization proceed rapidly or gradually and was it driven by rare bright galaxies, or numerous faint ones ? In my talk I will present recent results from various efforts to observationally address the interplay between galaxies and the intergalactic medium. I will show evidence that ionizing photons escape from galaxies when the hottest stars are still shining, indicative of an important contribution of rare bursty phases of star formation to the reionization budget. I will also show the first JWST results from the EIGER survey, which is a large program that is blindly searching for emission-line galaxies in the foreground, vicinity and background of the brightest quasars in the early Universe (z about 6-7). I will show how we confirmed redshifts for >100 galaxies in the first 500 Myr or so of cosmic time, and simultaneously measured the mass - metallicity for the first time using efficient NIRCam grism observations. We demonstrate that Lya transmission spikes in the quasar spectrum around redshift 6 are detected preferentially at characteristic distances of a few cMpc from galaxies. This is interpreted as the result of the competition between the high density of gas in the immediate vicinity of the ionizing sources and the drop off with distance in the intensity of the dominant local ionizing radiation field as the cosmic ionizing background weakens. Finally, I will highlight how to address current limitations in measuring the mass dependence of the escape fraction of ionizing photons with new and future VLT programs, and how the MUSE eXtremely Deep Field already provides a glimpse of how we can obtain tomographic studies of (re)ionized bubbles at the end stages of cosmic reionization with future ELT spectroscopy.
Friday 16 December 2022
Title : Past, present and future of integral field spectroscopy III. Example of scientific impact : MUSE spectroscopic deep fields
Speaker : Roland Bacon (CRAL — internal seminar)
Abstract :
This last lecture focuses on the main scientific outputs of the MUSE GTO deep-fields in the CDFS area, in particular the study of the population of low-luminosity lyman-alpha emitters, the first individual detection of lyman-alpha haloes and their properties and the detection of the cosmic web in emission and its possible implication for the existence of a population of ultra-faint galaxies.
Friday 9 December 2022
Title : Past, present and future of integral field spectroscopy II. The second generation and future prospects : MUSE, BlueMUSE and the Wide Field Spectroscopic Telescope
Speaker : Roland Bacon (CRAL — internal seminar)
Abstract :
This second lecture presents the second generation integral field spectrograph MUSE : the goal of the project, its study and realisation, its technical and scientific achievements and the lessons learned. The next part present two mid- and long-term projects : (i) BlueMUSE for the VLT, a successor of MUSE tuned for the blue wavelength range and higher spectral resolution, and (ii) the Wide Field Spectroscopic Telescope, a vey ambitious project of a wide-field 10m-class facility with advanced multi-object and integral field spectroscopic capabilities.
Friday 25 November 2022
Title : Past, present and future of integral field spectroscopy I. Birth of the concept and first generation : TIGER, OASIS and SAURON
Speaker : Roland Bacon (CRAL — internal seminar)
Abstract :
The first lecture presents the historical development of integral field spectroscopy (IFS). The introduction sets the scene, the circumstances that led to the invention of the concept and the realisation of the first prototype named TIGER. The IFS concept is then presented in more detail with emphasis on the main technologies : fibre, lenslet and image slicer. The development of IFS for adaptive optics is then presented with the OASIS instrument. The last part details the SAURON project : the instrument, the team and the survey. For each instrumental realisation discussed, I will present the instrument impact and some example of scientific results. I will also try to draw the lessons learned from this various developments.
Friday 18 November 2022
Title : From galactic dynamics to star formation : what sets the star formation rate ?
Speaker : Noé Brucy (CEA Saclay)
Abstract :
Star formation is a key process to understand the dynamic of the gas in the interstellar medium.
However the conversion of gas into star is a slow and inefficient process, and the observed star formation rate lie two orders of magnitude below what a gravity-only model would predict.
In this talk, I will present numerical simulations at the kiloparsec and the galactic scales with the aim of determining what are the main processes responsible for slowing down star formation.
I will focus on determining the respective roles of stellar feedback, large-scale turbulence and magnetic field, for Milky-Way like galaxies up to gas-rich galaxies typical of redshift z 1.
We will first see why models of an isolated region of a galaxy including only stellar feedback are not able to reproduce the Schmidt-Kennicutt relation and how these models can be modified to include the turbulence generated by motions at the galactic scale.
Then I will discuss ongoing effort to quantify and qualify the generation of such turbulence.
Thursday 21 October 2022
Title : Understanding the magneto-rotational evolution of stars
Speaker : Antoine Strugarek (CEA Saclay)
Abstract :
The magnetic field of our Sun is generated through an internal dynamo process leading to a cyclic variability of about 11 years. This cyclic field shapes the environment of our star and determines the connectivity in the heliosphere from the photosphere to the Earth orbit and beyond. The Sun has probably not always possessed such a cyclic field, nor should it keep it as it ages. Indeed, stars like the Sun spin down as they age, which greatly influences the dynamo processes they are able to sustain.
I will present ongoing efforts to model stellar dynamos in an evolutionary context. Based on numerical studies, we have recently proposed a unified view of the types of cycles solar-like stars are able to sustain. These results are confirmed using both simplified, mean field approach as well as 3D turbulent numerical simulations using various approximations. They show a tight link between the large-scale rotation profile within solar-like star, and the type of cyclical dynamos they are able to sustain. In this context I will detail the expected energy budget of such stars, and its implication for stellar dynamos. I will finally end with recent efforts where we start coupling dynamo action acting over decadal timescales with the response of the environment of the star, its dynamical corona.
Tuesday 18 October 2022
Title : PhD defence : Simulations of the circum-galactic medium - a comparative study of subgrid models for star formation and their feedback
Speaker : Maxime Rey (CRAL)
Abstract :
Over the last decades, our knowledge of galaxy formation has significantly advanced, with numerical simulations producing galaxies matching observational properties. In our current understanding of galaxy evolution, feedback processes hinder star formation and regulate the growth of galaxies. For low-mass galaxies, the expected origin of feedback lies mainly in supernovae, stellar winds, radiation pressure and photoionisation. These phenomena can heat the gas, disrupt molecular clouds and eject gas out of the galaxy. However, the inner workings of such processes are not entirely understood, and a new generation of simulations with increasing resolutions aims to tackle this challenge. These simulations rely on subgrid models describing unresolved phenomena at the resolution scale. However, distinct models following different physics can produce similar galaxies. A new observable beyond galactic properties is needed to lift this degeneracy, and a suitable place to do so is the medium closely surrounding the galaxy, the circum-galactic medium (CGM). This medium is difficult to model and highly sensitive to feedback processes. Indeed, the CGM hosts cosmic inflows fuelling the galaxy and outflows starving it, making it a medium driven by complex dynamics and an intricate multiphase thermal content.
I aim to test different accepted subgrid models against such a constraint. To this aim, I run zoom-in cosmological simulations of the same galaxy relying on distinct subgrid models, which I calibrate in stellar mass. I then post-process the simulations to obtain the ionisation state of the CGM and make mock observations of this medium through column densities. I find that despite having formed a similar stellar mass with each model, the content of their CGM exhibits notable differences, as well in the amount of gas as in the nature of this gas. This diversity translates in the simulated column densities, but some models are still degenerate, and the main discrepancy I find is between the simulated and the observed column densities. Two factors cause this discrepancy. The first is that outflows are not ejecting enough enriched gas in the CGM. The second is that the simulated ionisation fractions are likely incorrect, potentially due to inaccurate modelling of the thermal state of the CGM.
Friday 30 September 2022
Title : PhD defence : Cosmological simulations of cosmic rays and the Epoch of Reionisation
Speaker : Marion Farcy (CRAL)
Abstract :
To understand how our present-day Universe came into being, we have to look back to a few hundred thousand years after the Big Bang, during the Epoch of Reionisation. This epoch, which marks the last major transition of the Universe from neutral to ionised, involves the formation of the first stars and galaxies and the radiation from those galaxies ionising the intergalactic medium. Thanks to numerical simulations, it is now established that feedback - physical processes in galaxies that regulate their growth - is an important ingredient of the galactic ecosystem. In this thesis, I investigate the effects of cosmic ray feedback, cosmic rays being charged particles accelerated to relativistic speed at shock fronts, e.g. in the remnants of supernova explosions. Using the Ramses code, I performed and studied the first radiation-magneto-hydrodynamical simulations of galaxies combining supernova, radiation and cosmic ray feedback. My results show that cosmic ray feedback increasingly regulates star formation at decreasing galaxy mass, and leads to more and colder winds up to larger distances than those driven by supernova and radiation feedback. When cosmic ray feedback sufficiently regulates star formation at high-redshift, it also strongly and unrealistically delays the reionisation of the inter-galactic medium, by suppressing ionising radiation escape fractions. Therefore, my thesis shows that the details of cosmic ray feedback play a large role in the Epoch of Reionisation and on galaxy evolution.
Thursday 29 September 2022
Title : Bridging Optical and Far-Infrared Emission-Line Diagrams of Galaxies from Local to the Epoch of Reionization
Speaker : Yuma Sugahara (Waseda University)
Abstract :
Recent optical and far-infrared spectroscopy has demonstrated that the ISM properties evolve along redshift, but the range of redshift was limited depending on the wavelengths. I will present the redshift evolution of ISM properties predicted from photoionization modeling of galaxy populations at z 0, 2, and >6. We aim to show the continuous evolution by bridging optical ([OIII]5007/Hb and [NII]6585/Ha) and FIR ([OIII]88um/SFR and [CII]158um/SFR) emission-line diagrams using photoionization models. Our models predict that the galaxy distributions evolve along redshift on all the diagrams ; e.g., [OIII]/Hb and [OIII]88/[CII]158 ratios continuously decreases from z > 6 to 0, indicating higher ionization parameters at higher redshift. We additionally find that some of the z > 6 galaxies exhibit high [OIII]88/SFR ratios in the FIR diagram. To explain these high ratios at z > 6, our photoionization models require a low stellar-to-gaseous metallicity ratio (Z*/Zgas) or bursty/increasing star-formation history. Our model predictions will be tested by on-going JWST observations.
Thursday 29 September 2022
Title : Unravelling the birth of stars and protostellar disks
Speaker : Asmita Bhandare (CRAL — internal seminar)
Abstract :
Magnetized, cold, dense molecular cloud cores provide the birth environment for stars and disks. In this talk, I will summarize results from numerical simulations used to probe the gravitational collapse process that involves the transition of an isolated molecular cloud core to hydrostatic cores with a surrounding disk. We use the PLUTO code to perform radiation (magneto-)hydrodynamic simulations using one- and two-dimensional (2D) grids to help understand the role of various microphysical processes during the embedded stages. These include the effects of resistive magneto-hydrodynamics, self-gravity, radiation transport, and an accurate gas equation of state (to model the influence of molecular hydrogen dissociation). I will also briefly discuss some ongoing work involving the treatment of both gas and dust dynamics to provide better constraints on the gas-dust interaction and its impact on initial conditions for disk and planet formation.
Friday 29 July 2022
Title : Status update on the Dark Energy Spectroscopic Instrument and first results from IGM-based cosmology
Speaker : Satya Gontcho A Gontcho (Lawrence Berkeley National Laboratory)
Abstract :
The Dark Energy Spectroscopic Instrument (DESI) is in its first year of its 5 years run of collecting data at Kitt Peak National Observatory. This multi-object spectrograph is putting together the largest 3D map of the observable universe. I will give an update on the DESI project along with a glimpse at early results related to the intergalactic medium probed by high redshift quasars.
Friday 8 July 2022
Title : Resolved stellar populations in nearby galaxies : star formation history
Speaker : Lidia Makarova (SAO)
Abstract :
The Nearby Universe within 10 Mpc is a unique laboratory for the structure and galaxy evolution study, since all galaxies in this volume of space are well resolved to individual stars using the Hubble Space Telescope (HST). In recent 20-25 years several large and successful observational programs on imaging of the nearby dwarf galaxies were performed at the HST using the ACS and WFPC2 cameras. One of the goals of these programs was to determine the structure of the nearby universe bymeasurements of a large number of accurate photometric distances using Tip of the Red Giant Branch Distance Indicator (TRGB). Thanks to these projects, about 500 nearby (mostly) dwarf galaxies have well-resolved HST/ACS images in at least two broadband filters (usually V and I), as well as accurate photometric distances. Bright stellar populations are resolved in detail in the studied galaxies. The obtained data allow us to calculate the quantitative history of star formation of nearby dwarfs using theoretical stellar isochrones and modeling the resolved stellar populations. We have studied a number of galaxies both within the Local Group and in nearest groups of galaxies (in particular, M81 and Cen A). Dwarf galaxies, located outside the galaxy groups (in the field), probably have a number of features in their star formation.
Wednesday 6 July 2022
Title : New catalogue of edge-on galaxies based on the Pan-STARRS survey
Speaker : Dmitry Makarov (SAO)
Abstract :
We present a catalogue of 16551 edge-on galaxies created using the public DR2 data of the Pan-STARRS survey. The catalogue covers the three quarters of the sky above Dec. = -30°. The galaxies were selected using a convolutional neural network, trained on a sample of edge-on galaxies identified earlier in the SDSS survey. This approach allows us to dramatically improve the quality of the candidate selection and perform a thorough visual inspection in a reasonable amount of time. The catalogue provides homogeneous information on astrometry, SEXTRACTOR photometry, and non-parametric morphological statistics of the galaxies. The photometry is reliable for objects in the 13.8-17.4 r-band magnitude range. According to the HyperLeda database, redshifts are known for about 63 per cent of the galaxies in the catalogue. Our sample is well separated into the red sequence and blue cloud galaxy populations. The edge-on galaxies of the red sequence are systematically Δ(g - i) ≍ 0.1 mag redder than galaxies oriented at an arbitrary angle to the observer. We found a variation of the galaxy thickness with the galaxy colour. The red sequence galaxies are thicker than the galaxies of the blue cloud. In the blue cloud, on average, thinner galaxies turn out to be bluer. In the future, based on this catalogue it is intended to explore the three-dimensional structure of galaxies of different morphologies, as well as to study the scaling relations for discs and bulges.
Friday 1 July 2022
Title : Mapping the Circumgalactic Medium and Intergalactic Medium Through Both Absorption and Emission
Speaker : Zheng Cai (Tsinghua University)
Abstract :
In this talk, I will present our survey of the massive overdensities and enormous Lya nebulae in the peak of cosmic star formation and AGN activities (z=2-4). I will present our discovery of a sample of massive overdensies at z=2.3 by utilizing Subaru/HSC, Keck, CFHT and ALMA, selected using the groups of strong Lya absorption. The galaxy properties in these overdensities reflect the emergence of the environmental dependence of galaxy property. I will further discuss the discovery of a large sample of Enormous Lyman alpha nebulae in these extremely overdense fields at high redshift. Our Keck Cosmic Web Imager and ALMA observations suggest that these nebulae are unique laboratories to study the detailed modeling of how galaxies are fed by the intergalactic medium and circumgalactic medium. In the end, I will talk about the future prospects of how greatly improve our understanding of the structure formation in the next decade with LSST, CMB-S4 and a potentially new opportunity of a planned 6.5m spectroscopic telescope.
Thursday 30 June 2022
Title : Exploring the role of radiation pressure in star cluster formation with 3D radiation hydrodynamic simulations
Speaker : Shyam Menon (ANU)
Abstract :
The radiation pressure exerted on dust grains is potentially an important regulator of star formation in the interstellar medium, especially in denser molecular clouds that are optically thick to the UV reprocessed infrared radiation emitted by the grains. Studying this problem requires the use of 3D radiation-hydrodynamic (RHD) simulations, which solves the coupled equations of hydrodynamics and radiation transport. I will present VETTAM, a novel RHD method based on the Variable Eddington Tensor (VET) closure that is substantially more accurate than earlier methods. I will then present recent results using this code, wherein we quantified the impact of the dust-reprocessed radiation pressure on regulating the star formation efficiency and driving dusty winds in star clusters. Our results have implications for understanding star formation in super-star cluster forming environments and the possibility of radiation pressure-driven winds in galaxies and dusty AGN.
Friday 24 June 2022
Title : Cosmic Magnetism and Plasma Microphysics (or, I get by with help from my little friends)
Speaker : Matthew Kunz (Princeton)
Abstract :
The Universe is magnetized. While magnetic-field strengths of just 10^-18 G are required to achieve this both in our Galaxy and in clusters of galaxies, observations of Faraday rotation, Zeeman splitting, and synchrotron emission all make the case of ubiquitous μG fields. That these systems are not content with hosting weaker fields is surprising, at least until one realizes that the energy density of a μG field is comparable to that of the observed turbulent motions. It is then natural to attribute the amplification and sustenance of (at least the random component of) the interstellar and intracluster magnetic fields to the fluctuation (or "turbulent") dynamo. In this talk, we will explore the various ways in which plasma microphysics makes magnetic-field amplification in weakly collisional plasmas by macroscale turbulent motions both possible and efficient, with application to the intracluster medium of galaxy clusters. The predictions of this work are consistent with (i) deep Chandra X-ray observations of turbulence in the Coma cluster that suggest an anomalously low viscosity, (ii) recent LOFAR observations of diffuse radio emission in massive clusters when the Universe was only half of its present age, (iii) Faraday rotation maps of intracluster magnetic fields revealing structure on scales 1-10 kpc, and (iv) plasma kinetics measured directly by in situ spacecraft in the solar wind. Time permitting (as a bonus), I will advertise theory and simulation demonstrating how resistive tearing changes the geometry of turbulently amplified magnetic fields at large magnetic Reynolds numbers.
Friday 17 June 2022
Title : Uncovering Extremely Metal-Poor Galaxies Towards the Era of JWST Studies
Speaker : Yuki Isobe (University of Tokyo)
Abstract :
Primordial galaxies characterized by low metallicities and low stellar masses are important to understand galaxy formation, while it is hard to observe primordial galaxies at high redshifts due to their faintness. Complementarily, various studies have actively investigated local extremely metal-poor galaxies (EMPGs). EMPGs are expected to be good local analogs of high-z primordial galaxies because EMPGs have low metallicities, low stellar masses, and high specific star-formation rates. Using Subaru deep images, our EMPRESS project has successfully identified 20 EMPGs with very low stellar masses (10^4.2-10^6.6 Msun). Remarkably, our EMPGs include J1631+4426 with the lowest gas-phase metallicity (0.016 Zsun) identified so far. We find that J1631+4426 has a high Fe/O ratio ( 0.1) comparable to the solar abundance ratio, suggestive of the inclusion of bright hypernovae and/or pair-instability supernovae preferentially produced in a metal-poor environment. Our new IFU observations with R 7500 also uncover that EMPGs have low rotational velocities (5-21 km s-1) with respect to their velocity dispersions (19-33 km s-1), which can be explained by turbulence driven by inflow and/or outflow. These local EMPG studies shed light onto the early process of galaxy formation, providing a sneak peek of high-z primordial galaxy surveys in the forthcoming JWST era.
Friday 3 June 2022
Title : Adventures in Dusty Discs
Speaker : Sijme-Jan Paardekooper (Queen Mary University of London)
Abstract :
Planets form in discs of gas and dust around young stars. Somehow, dust particles have to grow from sizes typically found in the interstellar medium (sub-micron) to full planetary size. There are several challenges along the way : solid rocks tend not to stick together upon collision, and if they manage to grow to a metre in size they start to race towards the central star at breakneck speed (radial drift). This puts any growth mechanism under severe time pressure. I will discuss some of the possible solutions, the progress that has been made through simulations, and the numerical challenges involved.
Friday 20 May 2022
Title : Galaxy Evolution in Absorption — Beyond the Visible Tip of the Galaxy Iceberg
Speaker : Jens-Kristian Krogager (CRAL — internal seminar)
Abstract :
In this talk I will focus on how we identify very distant galaxies that are too faint to be seen in photometric surveys. These faint galaxies are the most numerous in the universe and are extremely important for our understanding of how galaxies evolve. I will present my work on using bright background sources to see the absorption fingerprints of galaxies. These fingerprints allow us to study the amount of gas and the production of heavy elements in galaxies. For the slightly more advanced part of the talk, we can even measure the temperature of the cosmic microwave background using this method.
Friday 6 May 2022
Title : Expanding on Hubble’s law
Speaker : Asta Heinesen (CRAL — internal seminar)
Abstract :
A century ago, Lemaître, Slipher and Hubble discovered an approximate proportionality law between distances and redshifts to astrophysical sources in our cosmic vicinity. This proportionality law is today referred to as Hubble’s law (or the Hubble-Lemaître law) and is central to much modern cosmological data analysis, where it is used to determine the expansion of our Universe. Hubble’s law might be viewed as an isotropic and lowest order Taylor series expansion of distances around the observer. When the Universe is not perfectly isotropic and when astrophysical sources span a large range of distances, Hubble’s law breaks down as an approximation for distances. I will describe how to expand on Hubble’s law in order to consistently take into account effects of inhomogeneity and anisotropy when analysing modern cosmological datasets.
Friday 15 April 2022
Title : Galaxies : from life to death
Speaker : Jérémy Fensch (CRAL — internal seminar)
Abstract :
This informal presentation will be split into two parts.
First, I will discuss the topic I presented on the national radio a few weeks ago : the story of a galaxy that we observed on the verge of dying. I will present you the data we obtained on that spectacular galaxy and explain how they forced us to go beyond the current paradigm on what we think could be killing galaxies.
Second, I will briefly introduce my current subject of research, which is to understand how galaxies formed their stars when the Universe had its peak in star formation. This cosmic period, which lasted a few billion years, saw the formation of around half the stars we observe today, but we do not understand yet what causes star formation to be so inefficient, and galaxies to have so much gas at that period.
I’d like to stress that this presentation will aim at being as accessible as possible to all the public at CRAL.
Friday 8 April 2022
Title : Object detection, characterization and reconstruction from faint signals in images : applications for direct imaging of circumstellar environments
Speaker : Olivier Flasseur (LESIA)
Abstract :
Astronomy is a field of study in which optical advances have driven the design of new generations of instruments always more efficient and dedicated to specific tasks. In particular, the detection of exoplanets and their characterization by direct imaging from the Earth is a hot topic. Beyond the detection of exoplanets, the reconstruction of circumstellar disks made of gas and dust is of primary astrophysical interest since exoplanets could form inside such structures.
The detection, characterization and reconstruction of the objects of interest remain very challenging due to the underlying low signal-to-noise ratio regime, thus requiring a fine processing of the data by dedicated and versatile algorithms. In this seminar, I will present some of the methods I have proposed in this field. The imaging challenges are formalized within an inverse problems framework. The main focus is put on the use of statistical and/or physics-based approaches to derive reliable and quantitative estimates. Information redundancies (e.g., temporal, multi-spectral) are also exploited. Robust processing strategies allow deploying the methods on massive data often corrupted by outliers. Besides, I will illustrate how deep learning techniques can push further the detection sensitivity for exoplanet imaging.
Almost all the developed algorithms are unsupervised : model’s parameters, weighting and/or regularization hyperparameters are estimated in a data-driven fashion making the methods efficient for the processing of real data of uneven quality. I will illustrate that this versatile framework allows addressing similar data-science challenges encountered in other applicative fields like microscopy.
Friday 1 April 2022
Title : Etching Glass : Hydrogen Fluoride in astrophysics
Speaker : Matt Lehnert (CRAL — internal seminar)
Abstract :
On behalf of the scientific counsel, I begin by explaining the nature and goals of the Internal seminar. I then discuss the molecule, hydrogen fluoride (HF), and why it is interesting for understanding the nature of molecular gas. At the end and very briefly, I discuss an observation of HF of a quasar and a vigorously star forming gas observed about 1-2 Gyr after the Big Bang. It is a result I found very unexpected. The talk, I hope, will be of interest to all members of the CRAL with some scientific background and will not require any particular expertise in molecular gas, HF, or general astrophysics. The idea is just to let people at CRAL know what I sometimes work on ! The total duration, talk + questions/interruptions, should be about 30 minutes.
Friday 25 March 2022
Title : Planet formation in stellar clusters
Speaker : Tom Haworth (Queen Mary University of London)
Abstract :
Star forming regions are hostile environments, with high stellar number densities and strong radiation fields. In addition to feedback from massive stars affecting the evolution of the star forming region itself, it can also affect the evolution of circumstellar discs, which could have implications for planet formation. In this talk I will review recent evidence for the environment influencing disc evolution and discuss some unsolved problems and future prospects.
Friday 11 March 2022
Title : Are magnetic fields shaping protoplanetary discs ?
Speaker : Geoffroy Lesur (IPAG Grenoble)
Abstract :
Over recent years, our understanding of protoplanetary disc (PPD) dynamics have changed dramatically. It is now believed that non-ideal MHD processes are playing a very important role in the formation and evolution of these objects. While a direct measurment of magnetic fields in PPDs is still lacking, I will show in this seminar that several observed features can easily be explained as a consequence of magnetised winds. In this context, I will revisit the problems of grain transport and accumulation, disc clearing and planet migration.
Friday 4 March 2022
Title : Resolving the Baryon Cycle in Emission and Absorption : First Insights from RAMSES-RTZ
Speaker : Harley Katz (Oxford University)
Abstract :
Galactic outflows are a fundamental component of the cosmic baryon cycle. Current generation simulations struggle to reproduce many of the observed properties of galactic outflows such as cold gas outflows and extended Ha emission. This is likely due to poor CGM resolution, lack of time-varying radiation fields, and ignoring non-equilibrium effects. In this talk, I will present the GLACIER project, a suite of simulations run with the newly developed RAMSES-RTZ code that couples non-equilibrium metal chemistry to time-varying local radiation fields. I will demonstrate how spectral signatures of resonant (e.g. Lya, MgII) and non-resonant emission (e.g. Balmer & nebular) and absorption lines can be used to measure outflow rates, outflow metallicities, and gas column densities.
Friday 14 Jan 2022
Title : Studying the Universe with gravitational-waves sources
Speaker : Simone Mastrogiovanni (Paris Centre for Cosmological Physics, Laboratoire Astroparticule & Cosmologie, Université de Paris)
Abstract : Gravitational waves (GWs) are ripples in the spacetime, generated by extremely energetic astrophysical sources. It took almost 100 years since their prediction by A. Einstein in 1916 to detect them and, since the first detection in 2015, more and more astrophysical sources have been discovered through gravitational-waves. After three observing runs, the LIGO and Virgo collaborations have observed almost 100 compact binaries coalescences, including binary black holes and binary neutron star mergers. GWs observations can be employed to study many of the open questions in current physics such as : Is general relativity valid on cosmological scales ? Is there a massive graviton ? How fast the Universe is expanding ? What is the formation channel of binary black holes ?
In this seminar I will show how GW sources can address these questions and also potentially introduce systematic biases in this kind of studies. After reminding how GWs are generated from compact binaries and detected by ground-based detectors, I will highlight how astrophysical information on the source are imprinted in their GW emission. Then, I will show how GWs from compact objects provide a ’’standard siren’’, a new way to measure cosmic distances, which can be used to measure cosmological parameters, such as the Hubble constant. I will then discuss how GWs can be used to test General relativity, in particular to test the nature of Dark Energy and the presence of a possible massive gravition. I will conclude by showing how populations of GWs sources can be studied to infer the formation channels of compact binaries systems.
Friday 17 Dec 2021
Title : How diverse are the faintest dwarf galaxies
Speaker : Martin Rey (Oxford University)
Abstract : Recent advances in imaging capabilities have transformed our understanding of galaxies at the low surface brightnesses frontier, unveiling a new population of ever smaller "ultra-faint" dwarf galaxies. Such low-mass, feeble objects are highly sensitive to the physical processes that shape galaxies in our Universe, providing an ideal laboratory for testing galaxy formation models. This same sensitivity, however, also generates extended scatter in their properties, which is essential to quantify to interpret findings in the next generation of deep, wide sky, surveys (e.g. the Vera Rubin Observatory, the Euclid and Nancy Roman Telescopes).
To begin this quantification, I present results from "genetically modified" cosmological simulations of galaxies, which allow us to uniquely separate the role of cosmological assemblies and physical models in a given galaxy’s final observables. I will show how this combination of abilities enables us to extract the expected diversity in the stellar and gaseous properties of the faintest galaxies in our Universe, making testable predictions that can be harnessed to constrain interstellar medium models with near-future photometric and radio surveys.
Friday 10 Dec 2021
Title : Quasar feedback and the origin of extended Lyα glow in z > 6 quasars
Speaker : Tiago Costa (MPIA Garching)
Abstract : The detection of quasars out to z ≈ 7.5 shows that supermassive black holes with masses of ≈109 M☉ have already assembled by the time the Universe was only ≈ 680 Myr old. These observations strenuously test theoretical models of galaxy evolution, which have to explain how such rapid black hole growth comes about. I will start by reviewing results from state-of-the-art cosmological simulations that show that black hole growth to ≈109 M☉ can be accommodated by galaxy evolution models. These black holes, however, must evolve inside rare, massive dark matter haloes tracing extreme overdensities. Zooming-in on the quasar host galaxies, I will argue that the required rapid black hole growth is expected to ignite powerful quasar feedback in the form of large-scale outflows. I will illustrate how these outflows, which are characterized by a multi-phase structure resembling that of the interstellar medium, affect quasar environments in a myriad of ways. I will then demonstrate how the same cosmological simulations that succeed in reproducing ≈109 M☉ black holes also account for recent observations of bright, extended Lyα nebulae around z > 6 quasars. I will make the case that the uncanny match between theoretical models and observations is only possible if quasar feedback already operates efficiently in z > 6 quasars. I will conclude my talk by highlighting new theoretical insights into the nature of Lyα nebulae at z = 6, explaining their detailed observational properties, their dominant physical mechanism, and the potential to detect them at z = 7.5.
Wednesday 24 Nov 2021
Title : Exploring the sources of cosmic reionization with simulations
Speaker : Maxime Trebitsch (Kaptey Institute Groningen)
Abstract : Cosmic reionization is one of the last major milestones in the evolution of the Universe. It has only taken one billion years for the bulk of the hydrogen in the Universe to be fully ionized by the radiation produced by early galaxies and quasars. While significant progress has been made in the recent years, completing the census of these ionizing sources is still a major challenge on both the observational and theoretical sides.
In this talk, I will discuss how radiation hydrodynamical simulations can be used to study the properties of the first galaxies and AGN with a focus on their relative role in reionizing the Universe. Using highly detailed cosmological "zoom-in" simulations, I will first discuss the main mechanisms that simulations suggests help radiation escape from early galaxies, and then move to the role of AGN during the Epoch of reionization. I will then use larger-scale simulations to generalise these results to a statistically relevant sample of galaxies and AGN.
Friday 4 June 2021
Title : The Cosmic Baryon and Metal Cycles
Speaker : Céline Péroux (ESO)
Abstract : Characterizing the relationship between stars, gas, and metals in galaxies is a critical component of understanding the cosmic baryon cycle. We compile contemporary censuses of the baryons in collapsed structures and their chemical makeup and dust content. We show the following :
The mass density of the Universe is well determined to redshifts and shows minor evolution with time. New observations of molecular hydrogen reveal its evolution mirrors that of the global star-formation rate density, implying a universal cosmic molecular gas depletion timescale. The low-redshift decline of the star-formation history is thus driven by the lack of molecular gas supply due to a drop in net accretion rate related to the decreased growth of dark matter halos. The metal mass density in cold gas ( K) contains virtually all the metals produced by stars for . At lower redshifts, the contributors to the total amount of metals are more diverse ; at , most of the observed metals are bound in stars. Overall, there is little evidence for a “missing metals problem” in modern censuses. We characterize the dust content of neutral gas over cosmic time, finding the dust-to-gas and dust-to-metals ratios fall with decreasing metallicity. We calculate the cosmological dust mass density in the neutral gas up to . There is good agreement between multiple tracers of the dust content of the Universe.
Friday 21 May 2021
Title : Investigating the connection of galaxy clusters to the cosmic web
Speaker : Céline Gouin (IAS)
Abstract : Galaxy clusters represent the most massive structures in the Universe, and they are connected to cosmic filaments that funnel accreting material.
The large-scale environment of galaxy clusters is therefore ideal place for investigating the growth of massive structures and this influence of the cosmic web on clusters.
Using large photometric catalogues around 6400 SDSS clusters, I will first present my recent work on the distribution of galaxies to probe how they evolve as they infall into clusters.
I will show that inside galaxy clusters the galaxy distribution is mainly elliptical, whereas we detect filamentary patterns in the clusters’ outskirts.
Filamentary structures are mainly traced by passive galaxies suggesting that galaxies inside filaments are stoping to form new stars.
Secondly and using state-of-the-art cosmological simulation, I will present the influence of cosmic web on cluster structural and dynamical properties.
We will see that the connection of cluster to the cosmic web is correlated to the cluster dynamical state. It appears to be the sign of different mass assembly histories of clusters.
Whereas dynamically relaxed clusters are slowly accreting matter and weakly connected to their environment, unrelaxed one are still in formation phase and strongly affected by the infalling of materials from filaments.
Friday 30 April 2021
Title : [Extra-ordinary] Kinematics of the Circumgalactic Gas : How Do Galaxies Get Their Gas
Speaker : Stephanie Ho (Texas A&M Uni)
Abstract : Galactic disks grow by accreting cooling gas from the circumgalactic medium (CGM). Although decades of observations have demonstrated that galaxies need a continuous gas supply to explain the star formation history, direct observations of gas accretion onto galaxies remain sparse. We will present results from our survey of using background quasar sightlines to measure the kinematics of the cool ( 10^4 K) CGM of low-redshift, star-forming galaxies. In particular, we will show that although the inner CGM corotates with the galactic disk, the centrifugal force only partially supports the circumgalactic gas, implying that the angular momentum of the CGM delays accretion onto the disk. We will also present our analysis with the EAGLE cosmological simulation and focus on the circumgalactic gas kinematics. Our study with EAGLE will provide insight into interpreting our circumgalactic kinematic observations and understanding how gas feeds the galactic disks.
Friday 2 April 2021
Title : An intuitive parametric model for MHD turbulence, and simple applications to astrophysics
Speaker : Jean-Baptiste Durrive (KU Leuven)
Abstract : An analytical model for 3D incompressible turbulence was recently introduced in the hydrodynamics community which, with only a few parameters, shares many properties of experimental and numerical turbulence. In view of modeling astrophysical environments, we suggest a simple extension of this approach to compressible and magnetized fluids. Our model constitutes a complementary tool to numerical simulations, as it enables us to generate very quickly fairly realistic velocity and magnetic fields, the statistics of which are controllable with intuitive parameters. In this talk, I will present our model in easily understood physical terms, with examples of applications.
Friday 26 March 2021
Title : Cuspy Dark Matter Density Profile in Disk-like Galaxies at z \sim 1 ?
Speaker : Gauri Sharma (SISSA)
Abstract : It has been known for decades that the rotation curve (RC) of a galaxy can be used as a proxy for the enclosed mass and its underlying distribution. In the local Universe, several studies have shown that the RCs of star-forming disc galaxies rise sharply in the inner region due to baryon dominance, while they remain flat in the outer regions and do not show the expected Keplerian fall, indicating the presence of invisible matter, so-called ’Dark Matter’. During my talk, I will present an observational study of progenitors of star-forming disc galaxies at z \sim 1 with a currently available large sample (KROSS data). This study confirms that [1] the RCs of z \sim 1 star-forming disc-like galaxies are similar to local disc galaxies (arXiv:2005.00279), [2] the dark matter fraction in these galaxies is above 50% (publication is in peer review). Both pieces of evidence are unique and in tension with several previous studies. In short, the main difference between my study and others lies in the accurate 3D modelling of the kinematics (in 3D space) and the consideration of pressure support without anticipating velocity dispersion. Furthermore, I will also show that at the early stage of disc formation, when pressure dominates the dynamics of baryonic matter, the gravitational interaction between baryon and dark matter becomes too feeble to establish the scaling relations that can be observed in local galaxies, but not at high redshift. In addition, I will highlight our recent findings indicating that the progenitors of disc galaxies at z \sim 1 exhibit denser dark matter cores. The latter evidence requires a detailed understanding of the cosmological evolution of the distribution of dark matter in galaxies, most likely related to its nature or feedback processes, which could be an interesting topic of discussion among the researchers of Centrede Recherche Astrophysique de Lyon.
Friday 19 March 2021
Title : Gas dynamics in high resolution in the Galactic Center
Speaker : Anna Ciurlo (UCLA)
Abstract : The proximity of the center of our Galaxy offers a unique opportunity to observe the environment of a supermassive black hole with a resolution inaccessible anywhere else. The galactic supermassive black hole is not very bright. The largest reservoir of material that could trigger a more intense phase is a molecular gas torus at about 1 pc but is presently kept at a distance. Smaller-scale processes can also accrete material onto the black hole and potentially trigger short peaks in the activity. Here I present the structure and dynamics of the ISM from few parsecs to sub-parsec scales. To do so I used spectro-imaging data gathered by the VLT and Keck and an innovative method : a regularized 3D fit (CubeFit). In the area encircled by the molecular gas torus the strong UV field is supposed to dissociate molecular hydrogen (H2). However, CubeFit allowed me to detected H2 everywhere. These molecules might have migrated from the molecular gas torus or have been formed in the winds of evolved mass-losing stars in the region and be shortly dissociated by the strong UV radiation. In the central few tenth of a parsec, I highlighted several compact, dusty sources in orbit around the central supermassive black hole. These objects might be the residuals of binary systems that merged under the influence of the central black hole. Regardless of their origin these puffy objects can transport material to the black hole and cause an increase in its activity. Processes at different scales highlight the complexity and interconnection of the mechanisms in the Galactic Center and the study of the interstellar medium at both small and large scale represent an important piece in understanding the environment of a supermassive black hole.
Friday 12 March 2021
Title : Galaxies and the nature of dark matter & gravity
Speaker : Benoît Famaey (ObAS)
Abstract : The standard cosmological model, with its matter component dominated by cold and effectively collisionless dark matter, is extrememely successful at reproducing the vast majority of large-scale observations of the Universe. In this short review, we highlight a few challenges at galaxy scales and discuss the prospects and challenges at solving them for some of the main alternatives to cold dark matter. We will discuss in particular (and, of course, briefly) warm dark matter, fuzzy dark matter, self-interacting dark matter, baryon-interacting dark matter, and modified gravity.
Friday 5 March 2021
Title : A new era of interferometry with GRAVITY
Speaker : Jean-Baptiste Le Bouquin (IPAG)
Abstract : The GRAVITY instrument has enabled major steps forward in infrared interferometry, by phase-referenced imaging at milli-arcsecond resolution, with a sensitivity increase by factor thousands, 30-100 micro-arcsecond astrometry, and few micro-arcsecond differential spectro-astrometry. This presentation highlights the game-changing results from the first three years of operation, from nearby exoplanets all the way to distant quasars and the precision tests of Einstein’s theory of General Relativity around massive black holes. The presentation includes an overview of the technologies behind the instrument and the on-going development of GRAVITY+ that will open all-sky, high contrast infrared interferometry.
Friday 26 February 2021
Title : Unveiling black holes through tidal disruption events
Speaker : Martina Toscani (UniMi)
Abstract : Black holes are among the most enigmatic and intriguing objects that populate our cosmos. They range from stellar black holes, originating from dying stars, to those monsters that reside in the cores of galaxies, with masses up to billions of times the mass of the Sun. Although we expect them to be widely spread through the Universe, they are quite difficult to see since black holes alone do not emit any radiation. What we need is a ‘cosmic warning light’ that may reveal their presence. This role can be played by tidal disruption events (TDEs). TDEs take place when a star orbiting around a black hole is fully or partially disrupted by the black hole tides. In this talk I will show how this occurrence is a treasure trove of physical information. First, I will describe general features of TDEs, focussing on the two kinds of radiation that they produce : gravitational and electromagnetic emission. Then, I will explore the possibility of detecting them individually combining the observations of future interferometers (e.g. LISA, TianQin, DECIGO) and telescopes (e.g. Athena, Lynx). Finally, I will show how TDEs are a powerful tool to detect the elusive intermediate-mass black holes that are assumed to live in globular clusters and to map their distribution up to redshift 3.
Friday 5 February 2021
Title : Unveiling the physical and chemical evolution of planet-forming disks : from ALMA to JWST
Speaker : Benoît Tabone (Leiden Uni)
Abstract : Planets form and obtain their composition in disks orbiting nascent stars (less than a few Myr). Therefore, the emergence of the diversity of exoplanets depends on the physical and chemical evolution of disks. ALMA, and the next generation of infrared facilities (JWST, ELT), is transforming the field by unveiling the emission of the gas and dust at disk scale (less than 50 au). In this seminar, I will present my on-going effort to analyse ALMA data and prepare the JWST observations of disks by combining complementary approaches, from detailed astrochemical modelling to numerical simulations. In particular, I will show that ALMA observations of bright outflows has brought first clues that accretion in disks is regulated by the launching of an MHD disk-wind. I will then discuss the impact of this kind of wind on the long term evolution of disks as probed by large ALMA and VLT surveys. Finally, I will discuss how to exploit the unique combination of high sensitivity and spectral coverage provided by JWST-MIRI to study the irradiation of disks.
Friday 29 January 2021
Title : Planet formation in multiple stellar systems
Speaker : Nicolas Cuello (IPAG)
Abstract : What is the impact of stellar multiplicity on planet formation ? This question is motivated by the fact that the process of stellar formation leads to a very high fraction of multiplicity. On top of this, planet formation occurs early on around young stellar objects. This leads to an unavoidable conclusion : planetary cradles, a.k.a protoplanetary discs, are deeply affected by the presence of nearby stars. Here, we will explore disc dynamics and planet evolution around binary and triple stellar systems, as opposed to single stars. First, I will briefly review the recent theoretical works on this subject. This will be illustrated with some representative examples of observed multiple stellar systems with discs and planets. Then, I will present our recent and ongoing modelling of protoplanetary discs in the presence of several stars, using 3D hydrodynamical simulations. Finally, based on this comprehensive picture, we will discuss the open questions in the field of planet formation in multiple systems.
Friday 11 December 2020
Title : Recent results from scattering transforms in astrophysics and cosmology
Speaker : Erwan Allys (LPENS)
Abstract :
New statistical descriptions related to the so-called scattering transforms recently obtained attractive results for several astrophysical applications. These statistics share ideas with with convolutional neural networks, but do not require to be learned, and allow direct characterization of interactions between scales in non-linear processes. In this talk, I will introduce these statistical descriptions, and present the results obtained on interstellar turbulence (characterization, classification and denoising) as well as on the cosmological density field (inference of cosmological parameters and statistical syntheses).
Friday 4 December 2020
Title : Investigating the physical processes driving the evolution of baryons in local and high-redshift low-metallicity galaxies
Speaker : Ambra Nanni (NCBJ)
Abstract : The chemical enrichment in the interstellar medium of galaxies is regulated by several physical processes : stellar birth and death, dust growth and destruction, galactic inflows and outflows. Understanding the interplay of such processes is essential in order to study galaxy evolution, the chemical enrichment of the Universe through the cosmic epochs and to interpret the available and future observations. Despite the importance of such topics, the contribution of different stellar sources to the chemical enrichment of galaxies, e.g. massive stars exploding as Type II supernovae and low-mass stars, as well as the mechanisms driving the evolution of gas, metal and dust grains, remains controversial. In this seminar, I will revise our current knowledge on these physical processes and the observational challenges. I will then present the results of a recent investigation focused on local low-metallicity galaxies for which the evolution of metals, gas and dust content has been studied. In particular, I will show how the comparison between model predictions and observations can allow us to identify the most relevant physical processes determining the chemical evolution of these systems. I will then discuss how the information derived for local low-metallicity galaxies can be employed to study Lyman-Break Galaxies at the epoch of reionization, which are often considered to be their high-redshift counterparts.
Friday 15 October 2020
Title : To be announced
Speaker : Laurence Denneulin (CRAL)
Abstract : To be announced
Friday 29 September 2020
Title : Pièges à poussière auto-induits dans les disques protoplanétaires : du rôle des lignes de gel à la formation de planétésimaux
Speaker : Arnaud Véricel (CRAL)
Abstract : La formation des planètes repose sur la croissance des grains micrométriques sur plus d’une dizaine d’ordres de grandeur dans les disques protoplanétaires. Pourtant, cette croissance est perturbée par des barrières à la fois théoriques et expérimentales. En effet, on a d’une part la friction aérodynamique entre le gaz et la poussière qui entraine la dérive radiale de la poussière vers l’étoile, qui devient maximale aux tailles millimétriques à centimétriques et résulte en l’accrétion rapide du disque. D’autre part, la vitesse relative entre les grains, qui leur permet de se rencontrer et de s’agglomérer, devient elle aussi importante pour les grains de tailles intermédiaires, ce qui les fait passer d’un régime de croissance à un régime de fragmentation.
Plusieurs solutions ont été proposées pour surpasser ces barrières, mais demandent souvent des conditions particulières pour se produire. Récemment, Gonzalez et al. (2017) ont proposé une solution qui s’appuie sur la croissance, la fragmentation et la rétroaction de la poussière, c’est-à-dire des éléments naturellement présents dans les disques. Ils ont appelé ce mécanisme "piège à poussière auto-induit". Ma thèse s’est concentrée sur ce mécanisme et son couplage à d’autres phénomènes couramment rencontrés dans les disques, le tout par le biais de simulations hydrodynamiques 3D avec le formalisme Smoothed Particle Hydrodynamics (SPH).
Je parlerai dans un premier temps de l’effet couplé de ces pièges avec la présence de fronts de condensation, appelés "lignes de gel". Ces dernières ont la particularité de modifier les propriétés de surface des grains, ce qui impacte la façon avec laquelle ils se collent et permet de concentrer la poussière à des endroits particuliers du disque.
J’explorerai également la possibilité qu’un disque contienne déjà une planète et parlerai de l’impact qu’elle aurait sur l’évolution de la poussière. Cet aspect est particulièrement important étant donné les observations de plus en plus fréquentes de disques et de leurs structures de poussière, dont il faut pouvoir expliquer l’origine. Je me pencherai plus précisément sur le cas du disque autour de l’étoile HD 169142, qui pourrait porter les traces de la croissance de la poussière autour d’une planète.
Pour finir, je présenterai mes travaux de développement d’outils numériques, notamment avec l’implémentation d’un modèle de croissance et de fragmentation de la poussière dans le code SPH public Phantom et son couplage avec le code de transfert radiatif Mcfost. J’ai utilisé ce code SPH moderne et modulaire pour effectuer des comparaisons avec des résultats connus et confirmer leur validité, notamment pour le cas de ces fameux pièges à poussière auto-induits. L’implémentation de ce modèle pourra servir à la communauté dans le cadre d’autres études de la croissance et de la fragmentation autour de systèmes variés et dans la production d’observations synthétiques.
Friday 24 September 2020
Title : To be announced
Speaker : Ugo Lebreuilly (CRAL)
Abstract : To be announced
Friday 21 September 2020
Title : To be announced
Speaker : Guillaume Laibe (CRAL)
Abstract : To be announced
Friday 18 September 2020
Title : Grain growth by the Galerkin method for the formation of planets
Speaker : Maxime Lombart (CRAL)
Abstract : Recent spatially resolved observations of protoplanetary discs have revealed the presence of structures with complex morphologies, indicating the probable presence of protoplanets inside. How could the grains in the disc have formed these structures ? How will they evolve ? To answer these questions, we need to understand how grains grow inside the disc. However, numerical resolution of the coagulation equation by traditional methods requires prohibitive sampling, making it impossible to integrate it into a three-dimensional hydrodynamic code.
The goal of this thesis is the development of an algorithm to solve the coagulation equation while respecting the constraints guided by astrophysics : rigorous conservation of mass while keeping a precision lower than the hydrodynamic errors on a mass spectrum relevant to the observations. Sampling must be minimal to allow coupling with a hydrodynamic code. To do so, the coagulation equation is solved by an original numerical scheme based on the discontinuous Galerkin method and a high order integration. The efficiency of the method is demonstrated on the known solutions of the equation and transferred to the context of planet formation.
Work carried out in parallel with this study will be presented. Firstly, an analysis of extra-solar planet observations made with the SPHERE/VLT instrument. Secondly, the numerical resolution of stochastic equations to understand grain sedimentation in turbulent disks.
Friday 16 September 2020
Title : Dynamique des fluides astrophysiques pour la formation des étoiles
Speaker : Benoît Commerçon (CRAL)
Abstract : Les étoiles fascinent, d’abord par l’émerveillement que procure l’observation d’un ciel nocturne à l’œil nu, et plus encore aujourd’hui grâce aux formidables images que nous apportent les télescopes. Pour les astrophysiciens, elles sont au cœur des recherches sur l’évolution des structures dans l’Univers, et leur étude engage une physique très riche. Les étoiles sont la partie visible des structures dans l’Univers et forment en leur sein les éléments lourds, en particulier le carbone, l’azote ou l’oxygène, essentiels à l’apparition de la vie. Au cours de leur évolution, depuis leur formation jusqu’à leur fin de vie, les étoiles sont au cœur des mécanismes qui régulent l’évolution des structures cosmiques, des amas de galaxies aux systèmes planétaires. Cependant, la formation des étoiles demeure encore aujourd’hui mal comprise, même si les grands principes sont bien établis depuis plus de 50 ans.
Le but de mon travail de recherche est de contribuer à une meilleure compréhension de la formation des étoiles, élément central du cycle de la matière interstellaire. Mon travail s’appuie sur le développement de modèles numériques pour les fluides astrophysiques, appliqués à la formation des étoiles, à la fois de faible masse et massives, ainsi qu’à la dynamique du milieu interstellaire. Les résultats de ces expériences numériques sont confrontés aux observations grâce à la production d’observations dites synthétiques, qui représentent l’observation des objets astrophysiques simulés dans des conditions réelles.
Friday 3 July 2020
Title : Biographie d’Adolphe Gouhenant
Speaker : Emmanuel Pécontal (CRAL)
Abstract : En 1832, une tour observatoire a été inaugurée au sommet de la colline de Fourvière, et ce monument dominait alors la ville de Lyon. En cherchant à connaître les travaux qui y avaient été menés, j’ai très vite compris qu’aucune observation astronomique sérieuse n’y avait été menée, mais son fondateur - Adolphe Gouhenant - m’a paru mériter des recherches. D’autres historiens s’étaient intéressé à lui en France, pour son action militante dans les premières années du communisme pré-marxiste, d’autres aux États-Unis, en tant que premier photographe du Nord Texas, mais aucune étude approfondie n’avait été conduite. En collaboration avec une de ses descendante au Texas, nous avons mené une enquête minutieuse de part et d’autre de l’Atlantique, révélant de nombreuses facettes de son personnage et ré-évaluant à la hausse son rôle dans le mouvement social des années 1840. Ce travail a conduit à la publication d’un ouvrage aux États-Unis, qui sera bientôt suivi d’une traduction française. J’en présenterai les grandes lignes dans ce séminaire, accessible à tous.
In 1832, an observatory tower has been erected at the top of the Fourvière hill, and at that time, this monument was overlooking the city of Lyon. While seeking which kind of research had been led in this observatory, I soon found that no serious astronomical observation was ever done there, but his founder - Adolphe Gouhenant - appeared to deserve some investigation. Historians in France have investigated his activism in the first years of pre-Marxist communism, others in the United States, his role as the first photographer of North Texas, but no comprehensive study has ever been made. In collaboration with one of his descendants in Texas, we led a meticulous investigation on both sides of the Atlantic, revealing many facets of his character and reassessing upwards his role in the socialist movement of the 1840s. This work led to the publication of a book in the United States, which will be followed soon by a French translation. I will summarize it in this seminar, accessible to all.
Friday 15 May 2020
Title : Topology and Geometry : Application to cosmological datasets
Speaker : Pratyush Pranav (CRAL)
Abstract : In the last couple of decades, topology and geometry have matured from purely theoretical fields towards strong focus on applicability in various research domains. The principal tool from Geometry has been the development of integral geometric quantifiers, viz. the Minkowski functionals. On the side of topology, a combination of Morse theory, homology and persistent homology, has enabled a new branch in data analysis called topological data analysis (TDA). The central tenet is based on the identification and assessment of geometrical and topological changes that occur in a manifold as a function of the excursion sets of the field. The topological changes are accounted for by tracking the creation and destruction of p-dimensional topological holes in a d-dimensional manifold. Intuitively, in 3 spatial dimensions, these changes correspond to creation and destruction of connected components, loops/tunnels and voids. The geometric quantifiers are associated with the notions of d-dimensional volume, area etc. These methods provide additional and complementary information with regards to traditional measures like n-point correlation functions. In the first part of my talk, I will present a non-technical summary of the methods.
In the second part, I will present two examples highlighting the application component. The first example concerns theoretical Gaussian random field models with power-law power spectra. We find that a topological characterization through Betti numbers and persistence diagrams provide information that is missed by traditional topological measures like the Euler characteristic and the more familiar geometric Minkowski functionals. The second example concerns the analysis of the topological characteristics of the temperature fluctuations in the Cosmic Microwave Background (CMB) from the temperature anisotropy maps measured by the Planck satellite. We find that the observed maps differ significantly from the simulations modeled as isotropic, homogeneous Gaussian random fields.
Friday 24 April 2020
Title : Influence of grain properties on dust evolution in protoplanetary discs
Speaker : Jean-François Gonzalez (CRAL)
Abstract : I will give an introduction on planet formation and highlight some of the current difficulties encountered by the models. I will then focus on how taking into account two kinds of grain properties can help us progress. I will first discuss the effect of their intrinsic density on the resulting chemical composition of the disc and its implication for current Solar System objects. In a second part, I will show that the porosity of grains can help to form planetesimals.
Friday 17 April 2020
Title : Where feedback fails : clues from the kinematics of disc galaxies
Speaker : Lorenzo Posti (ObAS)
Abstract : It is commonly believed that galaxies use, throughout the Hubble time, a very small fraction of the baryons associated to their dark matter halos to form stars. The fact that galaxies have typically very low baryon-to-stars conversion efficiencies both at low-mass & high-mass is typically interpreted as a consequence of stellar & AGN feedback respectively. While this is robustly established for the average galaxy population, is this true for all galaxies ?
In this talk I will show that if we can measure accurately the kinematics of the cold gas in disc galaxies, which traces very well the circular velocity, then we can measure these baryon-to-stars conversion efficiencies quite reliably for individual objects. High-quality radio interferometric data can be used to collect a fairly large (>100) sample of nearby discs of all masses, from dwarfs to massive discs, from which we find a couple of surprises. 1) the population of massive (> 10^11) discs has systematically higher efficiencies than spheroids of similar masses ; 2) the population of field, gas-rich ultra diffuse galaxies also has systematically higher efficiencies than a typical dwarf. These are two cases where AGN and stellar feedback, respectively, has failed to lower the baryon-to-stars conversion efficiencies of these galaxy population. I will finally discuss the implications of these results in our understating of galaxy evolution.
Friday 15 April 2020
Title : CANCELLED To be announced
Speaker : Anaëlle Maury (CEA)
Abstract : To be announced
Wednesday 18 March 2020
Title : CANCELLED [Extra-ordinary] Ramses with cosmic rays : anisotropic diffusion, streaming instability and shock-acceleration
Speaker : Yohan Dubois (IAP)
Abstract : I will detail the implementation of cosmic rays (CR) in Ramses and a few applications of CRs to galaxy formation problems. This numerical method of CRs is based on a fluid description of CRs. The anisotropic diffusion of CRs along magnetic field lines is modeled with an implicit solver. CR streaming instability (CRs streaming down their own gradient at the Alfven velocity) can be solved within the implicit diffusion solver. The so-called diffuse shock acceleration mechanism is captured with a shock finder algorithm performed on-the-fly that injects CR pressure at shocks with acceleration efficiencies depending on the Mach number, the pre-shock fraction of CRs, and the magnetic obliquity. A few applications of the diverse elements of the CRMHD description will be explored : structure of the interstellar medium, supernova remnants and the formation of large-scale galactic winds.
Friday 13 March 2020
Title : Assembling the Milky Way
Speaker : Florent Renaud (Lund Obs.)
Abstract : In the Gaia era, and with more and more complete surveys, the assembly of the Milky Way is being scrutinized with an always increasing accuracy. The kinematic and chemical properties of stellar populations are analyzed with the aim of reconstructing the formation history of the Galaxy. In this context, a major topic of interest is the role of galaxy mergers in driving the observed diversity of stellar populations. A number of cosmological simulations are currently attempting to retrieve these properties to propose an holistic formation scenario of our galaxy. In this talk, I will present a new cosmological zoom-in simulation at parsec resolution. I will use it for a detailed analysis of the signatures of mergers in the chemical abundances of the stellar populations, as opposed to in situ formation and intrinsic evolution. I will particularly present a new scenario for the simultaneous formation of the outer and inner thin disks.
Friday 6 March 2020
Title : Calibration of the tip of the red giants branch using GAIA DR2
Speaker : Dmitry Makarov (SAO)
Abstract : The Tip of the Red Giant Branch (TRGB) is proved as an excellent distance indicator, based on old stellar populations. GAIA data release 2 allows us to calibrate a zero point of TRGB using direct distance measurements of red giants in our Galaxy. In this work we show how to construct the color dependence of TRGB using data of SDSS and Pan-STARRS surveys. The main uncertainty is associated with the transformation of photometric systems, the accuracy of parallax measurements and the correction for Galactic extinction.
Friday 28 February 2020
Title : The Angular Momentum of the CGM in Illustris+TNG
Speaker : Daniel DeFelippis (Columbia Univ.)
Abstract : How disk galaxies form is still a crucial question in observational astrophysics. While disk properties must depend on their angular momentum, the complex kinematic coupling between the galaxy’s angular momentum and that of its surrounding material - commonly called the circumgalactic medium (CGM) - has been relatively unexplored. In this talk, I will present a characterization and analysis of the angular momentum of a large sample of galaxies’ CGM from the Illustris and IllustrisTNG cosmological simulations. I will describe angular momentum properties of the CGM for different halo masses, at individual redshifts, and over time as CGM gas is accreted onto galaxies and becomes stars. I will show that the angular momentum of galaxies and their CGM are correlated in large part due to a stellar feedback-driven baryon cycle.
Friday 21 February 2020
Title : Bilan du projet CALENDS (Clusters And LENsing Distant Sources)
Speaker : Johan Richard (CRAL)
Abstract : Je passerai en revue les résultats majeurs obtenus au cours du projet CALENDS, qui a impliqué un groupe de chercheurs au CRAL de 2013 à 2019. Le projet a consisté à exploiter le potentiel des amas de galaxies massifs comme télescopes gravitationnels pour étudier l’Univers distant. Une première étape a consisté à améliorer la calibration des modèles de masse d’amas massifs, notamment dans le cadres des ‘Champs Frontières’ du télescope Hubble. Ces amas ont par la suite permis de contraindre les sources très distantes de l’époque de la réionisation, ainsi que la structure interne des galaxies à tous redshifts pour tester les modèles de formation et d’évolution des galaxies.
Friday 31 January 2020
Title : Simuler un trou noir et son écharpe de plasma : de la parallélisation à
la GéPUfication d’un (vieux) code
Speaker : Emmanuel Quemener (Centre Blaise Pascal)
Abstract : Partout, on parle d’augmentation de puissance de calcul des ordinateurs mais comment et en quoi ces capacités ont-elles évolué ces 30 dernières années ?
Quoi de plus illustratif que de chercher à "retrouver" par la simulation une image comparable à celle qui a inondé nos médias en ce printemps 2019 : celle d’un trou noir et son écharpe de plasma.
Partant du premier article représentant un trou noir, un programme de 25 ans d’âge sera parallélisé en OpenMP, OpenCL puis CUDA, et enfin exécuté sur 30 différents "moteurs de traitement de l’information", des processeurs, classiques, graphiques et éphémères : ceux qui existaient il y a 30 ans, ceux qui émergeaient il y a 10 ans et ceux d’aujourd’hui.
Ainsi, entre l’antique 80386SX d’il y a 30 ans et l’atomique Tesla V100, c’est un milliard de gain pour seulement 100 fois sa consommation électrique. Et tout ça, grâce l’intégration de ce "vieux code C" directement dans du code Python...
Friday 24 January 2020
Title : Misalignments, precession and shadows in protoplanetary discs
Speaker : Rebecca Nealon (Leicester Univ.)
Abstract : Recent observations of protoplanetary discs have shown clear evidence of warps and misalignments, where the disc is best described with an orientation that changes as a function of the distance from the central star. A number of scenarios have been proposed to explain the origins of these misalignments, including the presence of a hidden companion on an orbit that is misaligned with respect to the outermost disc. By introducing such a misaligned body we need to consider the potential of disc precession, inclination damping and alignment time-scales. I will show how an understanding of these effects can allow us to infer what is happening in these systems with observed misalignments and the implications for their long term evolution. This tour will include applications to TW Hya (a rapidly moving shadow), misalignments induced by a flyby, HH30 (with a wiggling jet) and HD100453 (with a strongly misaligned inner disc).
Friday 17 January 2020
Title : Quenching star formation in massive galaxies
Speaker : Allison Man (Toronto Univ.)
Abstract : A fundamental question in galaxy evolution is how galaxies acquire diverse colours and morphologies. The current paradigm suggests that massive galaxies experienced accelerated growth in the early Universe and eventually quench their star formation. Exactly how galaxies quench is not well-understood. Many mechanisms have been proposed in the literature, yet a definite conclusion remains elusive. I will present an overview of the current state of the art and discuss future perspectives on solving this decade-old puzzle.
Friday 10 January 2020
Title : STRONG LENSING : Illuminating clusters of galaxies and the structures behind them
Speaker : Guillaume Mahler (Michigan Univ.)
Abstract : The strong gravitational lensing effect is a powerful technic to study both the deflector of light and the magnified sources behind it. In this talk, I will first review mass modelling approaches of galaxy clusters and present a state-of-the-art lensing model combining at the same time deep HST imaging and large spectroscopic coverage. Such a combination allows us to have a very refined mass distribution (dark matter and baryons) of a cluster of galaxies. An analysis of a larger sample of strong lensing clusters reveals that the lensing strength has a stronger correlation with the slope of the density profile rather than the total mass itself, indicating that mass is not the best criteria to select lenses in future surveys.
The second part of the talk will focus on the lensed universe. Strong lensing offers unique opportunities that have no match in blank fields, from the highly magnified galaxies at z 2, resolved by lensing down to tens of pc scales, to the high-redshift lensed luminosity function that reaches fainter and smaller sources,
Finally, I will discuss how future facilities such as JWST, LSST or Euclid combined with strong lensing will revolutionize our view of the universe in the near future.
Friday 20 December 2019
Title : MATISSE : The mid-infrared spectro-interferometric instrument of VLTI
Speaker : Philippe Berio (OCA)
Abstract : MATISSE (the Multi AperTure mid-Infrared SpectroScopic Experiment) is foreseen as a mid-infrared spectro-interferometer combining the beams of up to 4 UTs/ATs of the Very Large Telescope Interferometer (VLTI). MATISSE measures closure phase relations and thus offer an efficient capability for image reconstruction. In addition to the N band, MATISSE opens 2 new observing windows at the VLTI : the L and M bands, which all belong to the mid-infrared domain. Furthermore, the instrument offers the possibility to perform simultaneous observations in separate bands. MATISSE also provides several spectroscopic modes.
The unique performance of MATISSE is partly related to the existence of the four large apertures of the VLT (UTs) that permits to push the sensitivity limits to values required by selected astrophysical programs such as the study of Active Galactic Nuclei and protoplanetary discs. Moreover, the evaluated performance of MATISSE is linked to the availability of ATs which are relocatable in position in about 30 different stations allowing 200 meters baseline length. Key science programs using the ATs cover for example the formation and evolution of planetary systems, the birth of massive stars as well as the observation of the environment of hot and evolved stars.
During this talk, I will present the current status of MATISSE, its characteristics and its performances (measured during the commissioning phase). I will introduce the data processing software and I will also discuss the new mode of observation with the fringe tracker of GRAVITY : GRA4MAT. Finally, I will show some preliminary astrophycial results : first image reconstruction with MATISSE (FS Cma) and study of nano-diamonds in protoplanetary discs.
Friday 13 December 2019
Title : The challenge of indirect dark matter searches
Speaker : Angeles Moliné (UAM)
Abstract : A tremendous amount of gravitational evidence for the existence of dark matter has been accumulated from astrophysical and cosmological observations. In the past decades, a huge effort has been focused on understanding its nature. Dark matter is searched for using a variety of complementary probes. In this talk, I will give a brief overview of the search methods, focusing on indirect dark matter searches which rely on astrophysical observations.
Friday 6 December 2019 (HDR presentation)
Title : Low frequency radio properties of high energy sources, AGNs, Clusters
Speaker : Mamta Pommier
Abstract : The low frequency sky is dominated by rare faint unresolved radio emitting sources (including distant galaxies up to z=6, Epoch of Reionization) and by diffuse emissions from a variety of galactic and extragalactic objects. The radio emission in these celestial objects is believed to be due to synchrotron emission from relativistic electrons with the dominant decay mechanism being adiabatic expansion losses. However, for these population of faint and diffuse celestial sources, only tip of the iceberg has been discovered so far with the arrays operating at higher frequencies > 1 GHz. This limitation was mainly imposed due to lack of instrumental sensitivity as well as inherent spectral properties of these sources (favoring emission at lower frequencies < 1 GHz range). Thanks to the SquareKilometre Array (SKA)-Pathfinders operating at MHz-range like GMRT (150-1420 MHz), LOFAR (10- 250 MHz) and NenuFAR (10- 85 MHz), it is possible to investigate the radio properties of such faint diffuse and compact emission in a variety of celestial objects.
This HDR presents the author’s work in 3 different parts viz.,
(1)- Science projects- we summarize various new imaging results obtained with the SKA Pathfinders (GMRT, LOFAR) in last 15 years on Galactic (for e.g. microquasars, X-ray binaries, SNRs, high energy sources) and extragalactic (AGNs, cluster of galaxies) objects at low radio frequencies and discuss the emission mechanism of newly discovered compact and diffuse structures in them. We also present the results of HI emission line studies of the galaxies in cluster of galaxies environment using the Nancay Radio Telescope (NRT). These results concludewith the prospects for this work in the context of new instruments currently in place (NenuFAR) or in the near future, such as SKA projects led by the author and the collaborators. We also present the results of our study of multiwavelength (IR-IRAM/ALMA, Optical-HST/MUSE,X-ray-RXTE/ASM/Chandra) properties of Galactic and extragalactic objects in correlation with the low frequency radio data and present the synergy of LOFAR and SKA projects with upcoming multi wavelength facilities like WEAVE, Euclid, MSE, LSST, ATHENA etc..
(2)- Commissioning results and tools development for the users community on SKA- Pathfinders (LOFAR and NenuFAR) is presented. Early commissioning (beamformer and Imaging mode) results at very low frequencies (i.e. 45 MHz) with the French SKA-Pathfinder NenuFAR is also presented on A-Team calibrator (Cyg A, Cas A, Vir A) and LOFAR flux calibrators.
(3)- Teaching, pedagogic activities and training offered to national and international students is discussed towards the end of the talk along with the project to help the career of Women researchers.
Friday 29 November 2019
Title : Exoplanet detection in direct imaging by statistical learning of the non-stationary patch covariances : The PACO-based algorithms
Speaker : Olivier Flasseur (St-Etienne Univ.)
Abstract : We have recently introduced a new method dedicated to source detection from angular differential imaging (ADI) data : PACO (for PAtch COvariances). Data reduction in ADI is challenging because the faint point sources are hidden in a stronger nonstationary background (speckles) displaying strong spatial correlations. PACO learns locally a statistical model of the background directly from the data. This model captures short-scale spatial correlations up to a separation of ten pixels (i.e., within an image patch). The decision in favor of the presence or the absence of an exoplanet is then performed by a binary hypothesis test. PACO offers appealing characteristics compared to existing detection approaches. Since no image subtraction is performed, the photometry is preserved. PACO is completely parameter-free, including the computation of a detection map, its thresholding to extract meaningful detections, and the estimation of fluxes of the detected sources. Finally, the resulting detection maps are stationary and statistically well-modeled so that the false alarm rate, the probability of detection, and the contrast can be directly assessed without post-processing and/or Monte-Carlo simulations. We have shown using datasets from the VLT/SPHERE-IRDIS instrument that the proposed method achieves significantly better detection performance than current cutting-edge algorithms such as TLOCI or KLIP. We believe that these significant practical advantages should make PACO a method of choice for the analysis of ADI observations, in particular for large exoplanet surveys. We have very recently extended this algorithm to the joint processing of the different spectral channels of angular and spectral differential imaging (ASDI) data. The resulting algorithm, named PACO-ASDI , accounts for the spatio-temporo- spectral fluctuations of the data. Our tests conducted on several ASDI datasets from the VLT/SPHERE-IFS instrument show that PACO-ASDI also produces reliable detection maps and unbiased spectral energy distribution of the detected sources (with confidence intervals), outperforming the state-of-the-art exoplanet hunter algorithms.
Friday 22 November 2019
Title : NEAR : Imaging massive rocky planets in the habitable zone of Alpha Centauri
Speaker : Anne-Lise Maire (Liege Univ.)
Abstract : The New Earths in the Alpha Cen Region campaign is a 100-h observing program to image massive rocky planets in the habitable zone of the two stars of Alpha Cen, our nearest neighboring stars. This program is a collaboration between the Breakthrough Initiatives and ESO and was launched in 2016. To achieve the challenging goal of NEAR, the VLT MIR instrument VISIR was removed from UT3 for upgrades and installed at UT4 in order to couple it the Adaptive Optics Facility. The University of Liege provided an optimized vortex coronagraph for NEAR. After a successful commissioning in April and May 2019, the NEAR campaign was completed in May-June 2019. The campaign generated >6 TB of data, which are available to the community in the ESO archive. The data are currently under analysis by the NEAR collaboration. I will present the context of the project, my work on assessing the performance of the NEAR vortex coronagraph and on the data analysis, and the prospects for imaging Earth-mass planets around Alpha Cen from the ground in the near future.
Friday 25 October 2019
Title : TO BE RESCHEDULED Properties of galaxies at z $\sim$ 6 - 9 revealed by ALMA
Speaker : Takuya Hashimoto (Waseda Univ.)
Abstract : Understanding properties of galaxies in the epoch of reionization (EoR) is a frontier in the modern astronomy. With the advent of ALMA, it has become possible to detect far-infrared fine structure lines (e.g. [CII] 158 micron and [OIII] 88 micron) and dust continuum emission in star-forming galaxies in the EoR. Among these lines, our team is focusing on [OIII] 88 micron observations in high-z galaxies. After the first detection of [OIII] in the epoch of reionization (EoR) in 2016 from our team (z = 7.21 ; Inoue et al. 2016, Science 352, 1559), there are now more than ten [OIII] detections at z > 6 up to z = 9.1 (e.g. Hashimoto et al. 2018, Nature 557, 392 ; Tamura et al. 2019, ApJ, 874, 27). Interestingly, high-z galaxies typically have very high [OIII]-to-[CII] luminosity ratios ranging from 3 to 12 or higher, demonstrating [OIII] is a powerful tracer at high-z. The high luminosity ratios may imply that high-z galaxies have low gas-phase metallicity and/or high ionization states. These ALMA observations also allow us to detect / place upper limits on dust continuum emission. I will show the dust-to-stellar mass ratios of these very high-z galaxies. In addition to these ISM properties, I plan to present our detailed SED fitting results at z 7 - 9 taking ALMA data into account, which are useful to infer the star formation history of these objects.
Friday 18 October 2019
Title : Galaxy evolution : a gas perspective
Speaker : Jonathan Freundlich (Hebrew Univ.)
Abstract : As reservoir for star formation, gas is one of the main drivers of galaxy formation and evolution. With programs observing the molecular gas phase in typical star-forming galaxies at different epochs, I will present how the cosmic evolution of the star formation rate (SFR) and in particular its winding-down during the last ten billion years are mainly driven by the evolution of the molecular gas fraction. The depletion time associated to star formation indeed only weakly changes with redshift, both at galactic and at sub-galactic scales. I will show that the molecular gas content of star-forming galaxies during the winding-down of star formation does not seem to correlate with morphology, suggesting an ongoing supply of molecular gas to compensate for star formation while bulges grow. In contrast, molecular gas reservoirs can be dramatically depleted in extreme environments such as cluster centres. While large scale structure formation is primarily driven by dark matter (DM) dynamics in ΛCDM cosmology, gas processes can in turn affect the DM distribution at galactic scales. Using theoretical modelling and simulations, I will discuss how outflow episodes and gas density fluctuations induced by stellar feedback can expand both the DM and the stellar distributions, hence providing a simple understanding of the formation of DM halo cores and ultra-diffuse galaxies.
Friday 27 September 2019
Title : Resonant relaxation of stars around a supermassive black hole
Speaker : Jean-Baptiste Fouvry (IAP)
Abstract : In the vicinity of a supermassive black hole, stars move on nearly Keplerian orbits. Yet, because of the enclosed stellar mass and general relativity, the potential slightly deviates from the Keplerian one, which causes the stellar orbits to precess. Similarly, as a result of the finite number of stars, the mutual gravitational torques between pairs of stars also drive a rapid reorientation of the stars’ orbital orientation, much faster than the standard two-body relaxation driven by local scatterings. Overall, the combination of these two effects leads to a stochastic evolution of stellar orbital angular momentum vectors, through a process named ``resonant relaxation’’. Owing to recent developments in the diffusion theory of long-range interacting systems, I will show how one can fully describe such dynamics, in particular scalar resonant relaxation (relaxation of the norm of the angular momentum) and vector resonant relaxation (relaxation of the direction of the angular momentum vector). I will also highlight some astrophysical applications of these new methods, for example to understand the inefficiency of resonant relaxation to induce stellar tidal disruptions.
Friday 20 September 2019
Title : Two or three tricks from machine vision applied to the study of the magnetized interstellar medium
Speaker : Juan Diego Soler (MPIA)
Abstract : Machine vision is the set of tasks for gaining a high-level understanding of images sets, automating or enhancing the tasks that the human visual system can do.
In this talk, I will describe how we used basic techniques of machine vision for the analysis of the recent observations of the dust polarized emission with the Planck and BLASTPol instruments.
I will report how these techniques are providing a crucial insight into how magnetohydrodynamic (MHD) turbulence is structuring the interstellar medium (ISM).
Finally, I will show how the extensions of these machine vision techniques can be used to look for the imprints of the molecular cloud formation and destruction in the extended HI and 13CO observations from the THOR and GRS surveys of the Galactic plane.
Friday 13 September 2019
Title : Absorption spectroscopy as a unique probe of galaxy evolution
Speaker : Jens-Kristian Krogager (IAP)
Abstract : Our understanding of galaxy evolution cannot be complete without characterizing the ISM properties throughout cosmic time. In particular, the chemical enrichment of the ISM plays a crucial role in the cooling processes that lead to star formation. Moreover, the build up of metals in the ISM is related to the feedback from star formation and AGN. Hence, constraining the chemical evolution of gas in galaxies provides important clues for models and simulations of galaxy evolution.
I will demonstrate how, at cosmological distances, the ISM is best studied in absorption seen towards bright background sources. Since the detection of absorption systems depends on the gas cross-section and not on the luminosity of the associated galaxies, this technique allows us to probe much fainter galaxies than what is observed in flux-limited surveys at high redshift. In this talk I will present recent results highlighting the strength of absorption-selected galaxies as a cosmological tool. Lastly, I will discuss current issues and relate those to future missions which will be imperative for the advancement of the field.
Friday 26 July 2019
Title : Contribution of multipolar electromagnetic fields to the radio and high energy emission of pulsars
Speaker : Anu Kundu (ObAS)
Abstract : Studying the electromagnetic field around neutron stars is one of the vital methods to understand the physics of the pulsars. While major literature uses assumption of a standard centred dipolar electromagnetic field, recent studies have focused on including higher multipolar field components and have presented a more generalized picture for pulsars, in which the magnetic dipole moment is shifted off from the centre of the star. In this talk, the consequences of an off centred rotating magnetic dipole in vacuum will be discussed, by showing various characteristic features of magnetic field lines and pulsar emission. A broadband spectrum study of pulsar radiation will be laid out, by presenting maps of different emission regions of pulsars, distinguished on the basis of their frequency - the main aim of which has been to look for the evolution of the pulse profile with frequency. The results of this work (done as a PhD thesis) accompanied by necessary discussions to understand the theoretical models used and significant details of the numerical methods applied will be presented.
Friday 19 July 2019
Title : Gravitational Arcs : a closer look at typical z\sim1 disc galaxies
Speaker : Vera Patricio (DARK)
Abstract : I will present the analysis of the resolved properties of a sample of highly magnified gravitational arcs. These are typical z\sim1 disc galaxies that are gravitationally lensed by massive galaxy clusters, which allows us to probe their properties at spatial scales of a few hundreds of parsecs. I will particularly focus on properties derived using MUSE IFU data : the kinematics and ionised gas turbulence of these galaxies, as well as their metallicity. We model both lensing and observational effects to study the intrinsic velocity dispersion and metallicity gradient of these objects. Surprisingly, despite their high turbulence and large star-forming regions, typical of z\sim1 galaxies, the kinematics and metallicity gradient of these objects is quite similar to what is found in local (mature) discs.
Friday 5 July 2019
Title : Hydrodynamics and radiation from tidal disruption events
Speaker : Clément Bonnerot (Caltech)
Abstract : When a star gets too close to a supermassive black hole, it is disrupted by strong tidal forces in a tidal disruption event or TDE. Following this encounter, the stellar debris gets funneled to the vicinity of the compact object through multiple collisions with itself, causing the formation of an accretion disc. The associated dissipation results in a powerful multi-wavelength flare that represents a unique probe of the center of otherwise quiescent galaxies. I will present the first realistic simulation of this process, making use of a novel strategy to circumvent computational issues encountered by previous studies. This work sheds new light on the hydrodynamics at play during a TDE as well as the origin of the emerging radiation detected at an ever-increasing rate by transient surveys.
Friday 14 June 2019
Title : Dust Evolution from the Perspective of Nearby Galaxies
Speaker : Frédéric Galliano (CEA)
Abstract : A precise characterization of the grain properties is crucial for understanding the life cycle of the interstellar medium (ISM) and the evolution of galaxies. Nowadays, most of our knowledge of dust physics comes from studies of the Milky Way (MW). However, an increasing number of results on nearby galaxies provide unique discriminating constraints on fundamental grain processes. Indeed, nearby galaxies harbor a wider diversity of environmental conditions (metallicity, star-formation activity, etc.) than can be found in the MW. In particular, these nearby systems allow us to observe dust in extreme conditions, providing us valuable empirical information on grain evolution. They also constitute a necessary intermediate step toward understanding distant galaxies, as they are spatially resolved and have better wavelength coverage.
I will start with a general introduction, presenting the challenges in dust physics, and the recent advances in our understanding of the grain properties of the nearby Universe. I will then review several studies of the DustPedia project. This european collaboration aims to build a reference survey of ≈900 nearby, NIR-selected galaxies, observed with Herschel. I will detail the effort in modelling techniques we have carried out to provide the most accurate analysis possible of our data. I will finally present the various scaling relations and dust evolution trends derived among and within galaxies. I will discuss, in particular, the evolution of the dust-to-gas mass ratio and the fraction of aromatic feature carriers as a function of metallicity and star formation activity.
Wednesday 12 June 2019 [PhD Thesis]
Title : Low mass galaxies seen by MUSE and gravitational lensing
Speaker : Johany Martinez (CRAL)
Abstract : Galaxy formation and evolution is one of the most challenging mysteries in the observable Universe. In order to improve our knowledge in this field, the research make use of different observation programs to characterize scaling relations of physical properties, to better constrain and understand galaxy population at different stages of their lives throughout the history of the Universe. Since the past decades, studies are trying to extend those scaling relations in the parameter space. It is in this movement that this thesis project fits.
The Spectral Energy Distribution(SED) of high redshift galaxies contains the signature of physical properties such as stellar mass, SFR and extinction. In this work, we perform a SED analysis of magnified galaxies at z>3 using deep Hubble, VLT and Spitzer/IRAC images of the Frontier Fields galaxy clusters. Due to the size of the Kband PSF and specially IRAC PSF and the high density of bright cluster members, it is crucial to deblend Kband and IRAC images to get a reliable SED. We do this by automatically fitting the contaminating galaxies with GALFIT, using a custom Python script which accounts for the relative levels of contamination from each cluster member. We model the decontaminated SED using stellar population models.
We apply this method to derive SFR, masses and sizes of a sample of 63 galaxies at z>3 detected in the A2744 and MACS0416 fields, spectroscopically confirmed with MUSE. The very strong amplification of these clusters allow us to collect a robust sample of low-mass galaxies (10^8 M⨀), probing the low-luminosity part of scaling relations between stellar mass & size and stellar mass & stellar formation rate.
Friday 7 June 2019
Title : Interpreting the Dust Grain Asymmetries in Transition Discs
Speaker : Josh Calcino (Queensland Univ.)
Abstract : Observations of protoplanetary disks at multiple wavelengths have revealed spectacular disk morphologies, including spiral arms, circular and eccentric dust cavities, and azimuthally asymmetric dust horseshoes. All of these features can be caused by the interaction of companions, of planetary or stellar mass, with the disk. I will begin my talk by reviewing some of the key physical processes at work in these objects, and present some recent observations of protoplanetary discs.
I will then focus on a class of protoplanetary disks known as transition discs, which display large central cavities reduced in gas and dust. Several transition disks contain very large azimuthal asymmetries in their mm dust grain distribution. Traditionally these features have been interpreted as vortices induced by roughly Jupiter mass planets which orbit internal to the dust horseshoe. Using hydrodynamical simulations, I will show that these dust asymmetries can also result from the interaction between a binary star system and a circumbinary disc. I apply this model to a transition disc known as Ophiucus IRS 48, successfully explaining the dust horseshoe and peculiar features in the gas kinematics of this system.
Friday 3 May 2019
Title : The pupil-modulated Point-Diffraction Interferometer
Speaker : Nicolas Dubost (Durham)
Abstract : In high-contrast adaptive optics systems, non-common path aberrations between the wavefront sensing and the science paths need to be highly corrected. To perform an improved calibration, we have developed the Calibration and Alignment Wavefront Sensor (CAWS), a pupil-modulated point-diffraction interferometer (m-PDI). Here we give a brief introduction to the m-PDI concept and report the results of the integration of CAWS into CHOUGH, the Durham high-order adaptive optics system. Closed-loop experiments with static aberrations were performed with both monochromatic and polychromatic light. With monochromatic light, the residual error RMS is brought down to 55 nm across the entire pupil and 12 nm within a smaller area around the centre. Independently, PSF measurements showed Strehl ratio increases from 0.20 to 0.66. Preliminary broadband light tests with a FWHM of ∆λ=12% (80 nm) also showed a reduction of residual errors and a final Strehl of 0.74. Future developments and research prospects are also discussed.
Friday 26 April 2019
Title : Galaxy cluster cosmology from surveys of the hot Universe
Speaker : Nicolas Clerc (IRAP)
Abstract : Tracing the formation and evolution of the large-scale structure through its most massive constituents involves consideration of both galaxy cluster astrophysics (how do these objects form and take shape ? what are they made of ?) and population studies (how are they distributed in space ? how similar are they to each other ?) ; a specificity that is famously reflected in the problem of determining a cluster’s mass. I will present key aspects of the eROSITA all-sky survey and its forecasted 100 000 cluster sample, and show that its exploitation will not escape this necessity. In this context, association with large optical and radio surveys stands as a unique way to classify, select, measure objects for cosmology studies. On the other hand, detailed astrophysical studies of clusters enabled by the high-resolution X-ray spectrometer XIFU onboard the future ESA mission Athena will require well-understood samples to pinpoint the physics at play in their hot atmospheres. I will illustrate this reciprocity in perspective of measuring turbulent motions in the gaseous intra-cluster medium.
Friday 19 April 2019
Title : Three ways to quench a galaxy
Speaker : Sean McGee (Birmingham Univ.)
Abstract : The big evolutionary change in the life of a galaxy is its transition from a star-forming to a passive galaxy. In this talk, I will discuss three possible physical mechanisms which can induce this transition, each acting on a different regime of galaxy mass and large scale environment, but all of which are fundamentally related to the available gas content. I will discuss how satellite galaxies lose gas as they slam into the hot medium characterising massive halos, how massive galaxies are limited by their ability to accrete gas from the large scale environment and how intermediate mass galaxies fail to contain their gas after internal explosions.
Friday 12 April 2019
Title : From dust grains to planets : How the observations of substructures in protoplanetary disks has constrained the planet assembly ?
Speaker : Yann Boehler (IPAG)
Abstract : During the last 2-5 years, the advent of new observational facilities such as the Atacama Large Millimeter Array (ALMA), or the Sphere instrument on the Very Large Telescope (VLT), has drastically changed our vision of protoplanetary disks. Thanks to their very high spatial resolution and sensitivity, these instruments have shown that protoplanetary disks are not homogeneous structures as hypothesized before, but present numerous rings, spirals, and azimuthal asymmetries.
In this talk, I will summarize major results and will discuss the last advances in our understanding of rings formation, spirals, and dust asymmetries. I will deal particularly with the observations of rings, with a survey a 0.1’’ of 32 disks in Taurus that we recently performed, and spirals as observed in the disk around MWC 758. I will discuss if their presence can really be connected to planets or, on the contrary, be linked to local hydrodynamical instabilities, magnetic fields, and/or stellar flybys. However, planets might also be forming on at this moment in disks presenting dust concentrations. Using observations of MWC 758 and HD 142527, I will address current paths for the formation of these substructures and how they might lead to planetary embryos. I will finally propose future guidelines to make progress in the field.
Friday 5 April 2019
Title : The peak and edge of galaxy formation
Speaker : Oscar Agertz (Lund Observatory)
Abstract : In this seminar I will discuss the ongoing effort in modelling the formation of extended spiral galaxies like the Milky Way ; `the peak of galaxy formation’. I will review the advances made in the past few years within the galaxy simulation community, with an emphasis on the coupling between feedback from individual massive stars and the interstellar medium. I will present brand new results on the emerging chemodynamical structure of simulated galaxies, and how these hold the promise of unravelling the formation of the Milky Way in the era of large spectroscopic surveys. In the second half of the seminar, I will discuss the very ‘edge’ of galaxy formation, namely ultra-faint dwarfs. I will explain why they are possible test beds for star and galaxy formation physics, and what we have learned from new state-of-the-art models of such systems.
Friday 29 March 2019
Title : Structures, shapes and patterns in spatial data through probabilistic modelling and statistical inference.
Speaker : Radu Stoica (Lorraine Univ.)
Abstract : Spatial data are sets of observations made of elements having two components. The first component gives the coordinates where the observation took place. The second component, represented usually by a multi-dimensional real vector, represents the measures associated at the corresponding location. Digital images, environmental data in epidemiology or catalogues of celestial bodies in astronomy are some typical examples of spatial data.
The spatial character of the data induces a strong morphological component to the possible answers that may be given to questions arising from the data analysis. This explains why the question almost always arising is what is the pattern hidden in the data ?
The main assumption of our work is that the pattern we are looking for is made of random objects that interact.
Marked point processes are a probabilistic tool able to model random configurations of interacting objects. The main difficulty with these models is that they do not always exhibit a precise analytical form for their normalising constants. Hence sampling from such a probability density requires adapted MCMC simulation. Within this framework, statistical inference can be done,using methods such as the simulated annealing algorithm, the Monte Carlo maximum likelihood, permutation tests and bootstrap methods.
The aim of this talk is to introduce marked point processes and to illustrate their applications with examples and data sets coming from : cosmology, image analysis and environmental sciences.
Friday 22 March 2019
Title : A new data reduction algorithm for SPHERE : An inverse problem approach - a glimpse of forward model, a zest of calibration, a hint of robust penalization
Speaker : Anthony Berdeu (Laboratoire Hubert Curien)
Abstract : This talk presents a proposition for a new reduction algorithm for the IFS of SPHERE to extract the 2D+lambda data cube from the raw data of the instrument. Based on an inverse problems approach, it will be put into perspective with the current reduction pipeline. A part of the talk will be dedicated to the development of the forward model of the SPHERE instrument. Another part of the presentation will focus on the tuning of this forward model with the calibration files produced by SPHERE. Finally, the reconstruction algorithm will be presented. Based on a regularized robust penalization, it allows dynamically finding the defective pixels of the sensor. Preliminary results on extended objects as well as coronographic acquisitions will conclude the talk.
Friday 15 March 2019
Title : Maunakea Spectroscopic Explorer - the premier 10m class spectroscopic survey observatory for the next decades.
Speaker : Nicolas Flagey (CFHT) & Nicolas Martin (ObAS)
Abstract : The Maunakea Spectroscopic Explorer (MSE, formerly Next Generation CFHT) is an advanced project to profoundly transform the current CFHT into a survey telescope with a 10-meter mirror and a dedicated, heavily multiplexed, wide-field spectrograph with a wavelength coverage of 0.4-1.8 micron and multiple spectral resolutions. MSE is borne out as the response to the astronomical community’s need for a large aperture, dedicated, spectroscopic survey facility in synergy with imaging surveys (LSST, WFIRST, ...) and giant telescopes (GMT, ELT, TMT). The project is now entering preliminary design phase after successfully completing its conceptual design.
We will present the current architecture of the observatory as well as the expected performance characteristics of the project and describe the science that is driving this unique facility : unveiling the properties of the faint universe, whether they relate to the origin and diversity of stellar systems, Milky Way archaeology at the earliest time, galaxy evolution across cosmos times, or illuminating the dark universe. Finally, we will conclude with an overview of the project’s partnership and organization.
Friday 8 March 2019
Title : Spatially resolved emission line maps : a crucial ingredient to study giant star-forming regions at high redshift
Speaker : Anita Zanella (ESO)
Abstract : Giant star-forming regions ("clumps") with sizes < 1 kpc, masses 10^9 Msun and blue colors are ubiquitous features of z 2 galaxies. However, their formation phase has never been observed and their fate is highly uncertain. Are they born in-situ due to fragmentation of gas-rich disks or are they small companion galaxies that merged with the host ? Do they migrate inward and contribute to the formation of the galaxy bulge or are they rapidly disrupted by stellar feedback ?
To address these issues we are studying 50 clumpy galaxies at z 2, observed with ultra deep HST/WFC3 imaging and slitless spectroscopy : this approach allows us to select young and intermediate-age clumps, contrary to studies based on broadband imaging only. From spatially resolved emission line maps we discovered a bright, newly formed clump, in the very early phase of its collapse (age < 10 Myr). With a specific SFR 30 times higher than the one of the whole galaxy, it behaves like a mini-starburst, showing that in-situ violent disk instability can induce highly efficient star formation. Our estimate of the clumps’ lifetime ( 500 Myr) favours scenarios where they survive stellar feedback and suggests that their inward migration is a plausible mechanism to form the bulge of the galaxy. I will also discuss the analysis of intermediate-age clumps : their star formation rate, stellar mass, metallicity, and age distribution put further constraints on the origin of clumps, their lifetime, and their role in galaxy evolution.
Friday 22 February 2019
Title : Orbital and spectral characterization of the benchmark T dwarf HD 19467B.
Speaker : Anne-Lise Maire (Liege Univ.)
Abstract : The detection and characterization of substellar companions for which the age, luminosity, and mass can be assessed independently is critical to test and calibrate formation, evolution, and atmosphere models due to the degeneracies between these parameters. I will present an orbital and spectral analysis of the benchmark T dwarf HD 19467B recently detected with radial velocities and high-contrast imaging. This analysis is based on new data obtained with high-contrast imaging (VLT/SPHERE, VLT/NaCo), archival RV and photometric data (HARPS, Keck/HIRES, ASAS), and literature data. From the photometric data, we derived a gyrochronological age for the host star of 4.8-6.4 Gyr, which is significantly younger than age estimates from model isochrones (8-11 Gyr). A joint orbital fit of the imaging and RV measurements allows us to constrain the orientation of the orbital plane of the companion, its eccentricity (0.47-0.56), and its dynamical mass (57-66 Jupiter masses). Thanks to the new photometric measurements, the spectral energy distribution of the companion is extended to the K and L’ bands. The analysis of the spectrophotometric data confirms that the companion has a cool atmosphere with strong methane absorption bands. The measured properties of the companion are in good agreement with the expectations from evolutionary and atmospheric models for a cloud-free atmosphere.
Friday 15 February 2019
Title : Cosmic shear : from galaxy shapes to cosmology
Speaker : Nicolas Martinet (LAM)
Abstract : Weak lensing cosmic shear is one of the most powerful cosmological probes. In this presentation I will review the current state-of-the-art of such analyzes, from the accurate measurement of the galaxy image distortions up to the cosmological parameters’ inference. I will illustrate my argument with two recent studies : an observational application of a new cosmic shear estimator to the on-going Kilo Degree Survey (KiDS) and a simulation of the impact of undetected galaxies on shear measurement for the Euclid satellite.
Friday 8 February 2019
Title : Investigating Galaxy Evolution with Deep Learning
Speaker : Marc Huertas-Company (Paris Observatory)
Abstract : Deep learning is rapidly becoming a standard tool in many scientific disciplines including astronomy. I will review recent and on-going work on several applications of deep learning techniques to galaxy evolution related problems. I will show examples of how different network configurations can be efficiently used to classify galaxies into different evolutionary stages even when no apparent features are visible as well as to detect and measure substructures within galaxies such as bulges and clumps. I will also discuss usnupervised approaches based on generative models to compare numerical simulations and observations and detect anomalous objects. In my talk I will also try to show possible solutions to known limitations such as uncertainty estimation, small training sets and the “black box problem"
Friday 1 February 2019
Title : Gas fraction of galaxies and impact of mergers across cosmic times
Speaker : Jérémy Fensch (ESO)
Abstract : The recent acceleration in millimetre astronomy with ALMA has shown that the gas mass fraction of galaxies is an increasing fonction of redshift, until at least z=3. In particular, the bulk of star-forming galaxies typically have a molecular gas mass fraction above 50% of their total baryonic mass. We use parsec-scale hydrodynamical simulations of isolated and interacting galaxies with different gas fractions to study the impact of this sole parameter on the conditions of star and star cluster formation on galactic scales.
We first show that the high-gas fraction is responsible for the instability of the disk structure in gas-dominated disks, and lead to the formation of massive 10^8-9 Msun and long-lived ( >100 Myr) gaseous clumps. By testing different models of stellar feedback, including supernovae and radiative pressure from HII regions, we see that their evolution is more sensitive to the galactic shear and gas fraction than to stellar feedback. We further show that these clumps are the birth nests of young massive star clusters. This clumpy distribution of star formation is consistent with the observed UV rest-frame light clumpy morphology of galaxies at z 2, along with the obtained masses and survival times.
Although galaxy interactions are able to expel these star clusters to the halo of the galaxy merger remnant, and protect them from the galactic tidal field, we note that mergers of gas-dominated galaxies do not produce more stellar clumps, which suggests that major mergers are not a major channel of formation of star clusters at z > 2. This is mainly due to a saturation of the turbulence, whose initially high level can barely be increased by the interaction. This saturation also suggests that galaxy mergers are much less efficient at producing starbursts than they are at lower gas-fraction. This provides an explanation for the observed decreasing efficiency of merger-driven starburst with redshift.
Last but not least, detailed simulations of disk instabilities during interactions of intermediate-redshift type gas fraction disks (gas mass fraction of 30%, typical of z=0.7) show the formation of massive young stellar clumps. This collaborative work in progress could provide an explanation to the increased clumpiness observed for starbursting galaxies at this redshift in the COSMOS field, and opens prospects on the formation channels of the bulge and thick disc component of present-day galaxies.
Friday 25 January 2019
Title : Synthetic observations of early star formation
Speaker : Alison Young Gray (Exeter)
Abstract : The theory of how low mass stars form from dense molecular cloud cores is well established. A key stage of the process is the formation of the first hydrostatic core (FHSC), which is the first stable object predicted to form during the gravitational collapse of a pre-stellar core. There are many observations from far-infrared to radio of young, low-luminosity protostellar sources but the identification of the FHSC in nature remains elusive, even 50 years after it was first predicted from theory. Specific observational diagnostics are required in order to distinguish the FHSC from other faint objects. I will present synthetic spectral energy distributions (SEDs) and molecular line spectra of the FHSC that were produced from hydrodynamical models and, for the latter, from chemistry calculations. I will compare the results to observations of candidate FHSCs and discuss what these observations are likely to reveal about the kinematics and evolutionary stage of these very young objects and how this can inform the search for the FHSC.
Friday 18 January 2019
Title : Magnetic fields in galaxies : A new low-frequency radio continuum perspective.
Speaker : Sarrvesh Shridhar (ASTRON)
Abstract : Radio continuum emission is an excellent tracer of magnetic fields in galaxies. Historically, observations of galaxies at low radio frequencies (below 1 GHz) have suffered from poor angular resolution and sensitivity. This scenario is fast changing in the current decade with new radio telescopes like the LOw-Frequency ARray (LOFAR) and the Murchison Widefield Array (MWA). In this talk, I will provide an overview of how observations with these telescopes are improving our understanding of magnetic fields in galaxies. In the first part of the talk, I will explain the observational challenges associated with carrying out observations at low radio frequencies and present some of the new calibration algorithms that have been developed to overcome the this. In the second part, I will present results from our observations of a sample of nearby galaxies with LOFAR over the last few years.
Friday 11 January 2019
Title : News from our MUSE
Speaker : Roland Bacon (CRAL)
Abstract : Since a year MUSE is now running with its adaptive optics module, the Ground Layer AO, commissioned in 2017 and the Laser Tomography AO, commissioned this spring. In the first part of the talk, I will give some brief description of the system and the achieved performance. In the second part : I will give some highlights of the scientific results published recently with MUSE. Note : this talk is not a science talk strictly speaking but is more intended for all people broadly interested with MUSE. At the menu : a brief of technics, some spectacular images, a few plots and a pinch of scientific results.
Friday 21 December 2018
Title : Observing the multi-scale cosmic web at high redshift and quantifying its impact on galaxy assembly
Speaker : Clotilde Laigle (Oxford University)
Abstract : The cosmic web is the large-scale metric in which galaxies form and evolve. Significative evidences of the role of the cosmic web in driving galaxy properties have been measured from simulations and at low redshift from spectroscopic surveys. They support a picture in which the geometry of the large-scale environment drives anisotropic tides which impact both the dynamics and the assembly history of galaxies. But extracting the cosmic web from observed datasets is still a challenge, in particular at high redshift where large and complete spectroscopic surveys are extremely costly. At these redshifts, though, we expect a stronger dependency of galaxy properties on the geometry of the accretion, which makes this extraction pivotal to understand galaxy evolution.
I will give an overview of the current status of cosmic web analysis from high redshift observations, either photometric data or lyman-alpha forest surveys. While relying on a pilot study in COSMOS and forecasts from the simulated horizon-AGN lightcone, I will present results about the co-evolution of galaxies and the cosmic web and I will show how this study can be extended with future probes including LSST, Euclid, PFS and MOSAIC on the ELT.
Friday 14 December 2018
Title : The role of HI kinematics and HI column density on the escape of Lya photons in star-forming galaxies
Speaker : Lucia Guaita (PUC)
Abstract : The Lyman alpha radiation emitted from high-redshift sources is an excellent tool to study the properties of the galaxies themselves and of the medium they live in. By studying the spectra of Lya emitting galaxies at 4>z>2 from the VUDS and VANDELS surveys, we investigate how Lya equivalent width and Lya shape depend on the interstellar and circumgalactic medium properties. We find that the galaxies characterized by low HI column densities (NHI 10^18 atoms/cm2) experience stellar-driven outflows with velocities of the order of a few hundreds of km/s and present spatially concentrated Lya emission. In the galaxies characterized by large HI column densities (NHI > 10^18 atoms/cm2), Lya photons can be efficiently scattered even in the case of static media and can produce extended Lya halos. Dense environments can present a different amount and distribution of HI gas with respect to the field and could imprint the Lya emission shape. We find that Lya emitters generally avoid the most dense regions. However, the Lya emitters in dense regions tend to be characterized by low Lya equivalent width, low specific star-formation rate, to be more massive than those in the field, and their stacked Lya profile shows a dominant red peak plus a hint of a blue peak, not seen in the stack of the profiles of field galaxies. Also, there is evidence that the Lya emission from galaxies in dense environments is offset with respect to the UV continuum. Outflows with low expansion velocity and not-uniform geometries of the interstellar medium could explain the stacked Lya profile and the physical properties. Therefore, with the information provided by Lya, Lya emitters can give information also on the environment.
Friday 7 December 2018
Title : The many faces of stellar feedback and the roles in the galaxy baryon cycle
Speaker : Edouard Tollet (OBSPM)
Abstract : We investigate the role played by SN feedback in the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) simulation suite. The goal is to highlight and quantify the mechanisms that prevent star formation, or in other words, trying to explain why, at best, galaxies form only a few 10% of the universal baryon fraction times their halo virial mass of stars. SN feedback clearly plays a major role and can act in at least three ways : It can prevent the accretion of gas onto galaxies. It can eject gas from galaxies. It can slow down the conversion of gas into stars.
We observe in NIHAO dwarfs that prehemptive feedback is the dominant mechanism at low masses. In haloes with 10^10 solar masses or less,
moderate outflows can divert inflows at several virial radii and reduce the mass that accretes onto galaxies by one order of magnitude.
In contrast, this mechanism is of scarce significance in haloes with Mvir 10^11 and above.
Once accreted by a galaxy, gas forms stars in an amount related to the time it spends in the cold dense ISM.
Feedback can slow down star formation by reducing the time spent in that phase. This, too, can be done in more than one way : Gas can remain in the cold ISM but feedback can prevent cold neutral gas from condensing into molecular clouds ; Gas can remain within the galaxy but move back and forward from the cold ISM to the hot ISM several times ; Gas can circulate in a galactic fountain.
The paucity of cold dense gas is the primary reason why star formation is inefficient and sporadic in dwarf galaxies. Above Mvir 3*10^11 solar masses, the presence of a confining hot CGM prevents the development of outflows on scales larger than the galactic disc.
In massive galaxies, the main effect of feedback is to nourish the hot ISM. Less than 10% of the ejected gas mixes with the hot CGM and then cools back onto the galaxy, while this is the main reaccretion mechanism in semianalytic models of galaxy formation.
Friday 30 November 2018
Title : On the Role of Galaxies and Black Holes in Reionising their Intergalactic Environments.
Speaker : Koki Kakiichi (UCL)
Abstract : Study of reionization has now experiencing a surge of new insights, but the long-standing problem ‘what reionized the universe ?’ still remains unsolved. Hubble observations have placed a now-commonly-held view that the intrinsically faint galaxies are responsible for driving the reionization process, but with a fundamental assumption of large escape fractions >10%. A further puzzle comes from the recent deep spectroscopy of luminous galaxies and the spatial opacity fluctuation of the intergalactic medium (IGM) at z>5.7, which suggests a possibly important role of luminous systems and active galactic nuclei (AGN). To address these issues, we introduce a new Keck spectroscopic programme surveying 5
Friday 23 November 2018
Title : Understanding the structure of molecular clouds : Multi-line wide-field imaging of Orion B.
Speaker : Jan Orkisz (Chalmers University)
Abstract : The new generation of wide-bandwidth high-resolution receivers turns almost any radio observation into a spectral survey. In the case of wide-field imagind of the interstellar medium, such a wealth of data provides new diagnostic tools, but also poses new challenges in terms of data processing and analysis.
The ORION-B project aims at observing 5 square degrees of the Orion B molecular cloud, or about half of the cloud’s surface, over the entire 3mm band. The emission of tens of molecular tracers have been mapped, including CO isotopologues, HCO+, CN, HNC, N2H+, methanol, SO, CN...
Machine learning techniques have been applied to these maps, in order to segment the molecular cloud into typical regions based on their molecular emission, and to idenfify the most meaningful correlations of different molecular tracers with each other and with physical quantities such as density or dust temperature.
The spatial coverage, together with the spatial and spectral resolution, also allow to characterize statistically the kinematics and dynamics of the gas. The amount of momentum in the compressive and solenoidal (rotational) modes of turbulence are retrieved, showing that the cloud is dominated by solenoidal motions, with the compressive modes being concentrated in two star-forming regions - which is in line with the overall very low star formation efficiency of the cloud, and highlights the role of compressive forcing in the star formation process. The filamentary network of the molecular cloud also proves to have particluarly low densities, and is very stable against gravitational collapse and fragmentation, which also points at a young evolutionary stage of the filaments.
Friday 16 November 2018
Title : Metallicity gradients and the effects of diffuse ionized gas.
Speaker : Henry Poetrodjojo (ANU)
Abstract : The gas-phase metallicity of a galaxy is strongly affected by the processes that occur during the galaxy’s evolution. Gas inflows, galaxy mergers and galactic winds are a few examples of events that alter the spatial metallicity distribution. Measuring the metallicity of a galaxy therefore leads to strong constraints on its growth and formation. With the recent emergence of large IFU galaxy surveys such as CALIFA, SAMI and MaNGA, we can begin to probe the metallicity distribution of galaxies and disentangle degeneracies using large datasets. However, contamination by the diffuse ionized gas significantly affects metallicity measurements. At the low spatial resolution scales of these large IFU surveys, HII regions can not be separated from the diffuse ionized gas, leading to systematic errors in these measurements. We use high spatial resolution data from the TYPHOON survey to separate the HII regions and the diffuse ionized gas to understand the effects of spatial resolution and diffuse ionized gas contamination on measured metallicity gradients.
Friday 9 November 2018
Title : Transforming our understanding of the X-ray Universe : the Imaging X-ray Polarimetry Explorer (IXPE)
Speaker : Frédéric Marin (ObAS)
Abstract : 50 years after the pioneering experiments, X-ray spectroscopy and timing techniques can be considered as well established. Nonetheless, one prominent feature of X-ray light has not been explored as scrupulously as others : its polarization. Between 1980 and 2000, the instruments were not sensitive enough to go beyond the first X-ray polarimetric results acquired in the 70s but the development of new detection techniques in the early 2000s revived the field. The first X-ray spatial mission to fly a new generation polarimeter will be launched by NASA in 2021 and a couple of balloon-borne experiments are being considered. Following the increasing interest of the community to the unexplored phase space of X-ray polarization, I will present the discoveries X-ray polarimetry is about to make by focusing on the forthcoming NASA Imaging X-ray Polarimetry Explorer (IXPE). This mission will exploit the polarization state of light from astrophysical sources to provide insight into our understanding of X-ray production in objects such as neutron stars and pulsar wind nebulae, as well as stellar and supermassive black holes.
Friday 21 September 2018
Title : Pulsalys : research and industry
Speaker : Laetitia Roux (Pulsalys)
Abstract : Do you have research results with potential for a wider use ? Do you have questions about Intellectual Property of your research ? Do you think you may create a startup using your research results ? Pulsalys will present its activities and answer your questions about technology transfer, startup creation, etc. Pulsalys is a SATT, a Technology Transfer Office created in 2013 with public funding dedicated to technology transfer and Startup building. Our missions are : (1) to detect high-potential technological innovations from Lyon & Saint Etienne public research laboratories (2) to invest to transform early-stage technologies and invention into mature innovations that fit the market needs (3) to select the best partners for the innovations though co-development/licensing to a company or though startups/spinooffs creation. Key figures since inception : +500 detected inventions including 130 funded projects. 57 start-up businesses, generating +3M€ fundraising & 150 job creation. 46 signed patent license agreement. 150 IP assets including 118 filed patents. €14M invested.
Friday 14 September 2018
Title : Make your code famous ! (or at least discoverable).
Speaker : Alice Allen (ASCL)
Abstract : Source codes are increasingly important for the advancement of science in general and astrophysics in particular. Journal articles detail the general logic behind new results and ideas, but often the source codes that enable these results remain hidden from public view. In this presentation, I will discuss our recent study on the availability of source codes used for published research and how this affects the transparency and reproducibility of astro research. I will cover what the Astrophysics Source Code Library (ASCL, ascl.net) is, how to submit software to the resource, and the benefits of doing so. I will share what happens after software is submitted, how ASCL entries are indexed by ADS, the links between literature and software entries, and how an ASCL ID can be used for citing your code. I will cover good and bad ways to cite software, avenues for publishing software, and how journals are changing to include and recognize the contribution software makes to our discipline.
Friday 7 September 2018
Title : GMO-CLONES for an APC*
Speaker : Jenny Sorce (CRAL)
Abstract : To unveil the nature of 95% of the Universe, missions such as Euclid aim at reaching a few percent precision. In this quest for precision, tensions between the standard cosmological model and observations already arise : local and global H0 measurements are incompatible at more than 3-sigma, anomalies emerge within the CMB, etc. These tensions suggest that we should perhaps not be so quickly inclined to disregard our observational site as a bias factor : Accuracy is not Precision. Few percent precision and local biases are of the same order of magnitude. A precise mapping of the local distribution of matter is essential to properly account for these biases. Simulations constrained to resemble the local Universe constitute the tool of choice for such a mapping. I will review the genesis of such simulations as well as the first results that promise to tremendously impact our understanding of the local biases that will matter in future analyses. I will also present prospects for significant progress (the GMO-Constrained LOcal & Nesting Environment Simulations) to reach an Accurate Precision Cosmology*.
Tuesday 4 September 2018 [PhD Thesis]
Title : Evolution of the porosity of grains : a solution for planetary formation problems ?
Speaker : Anthony Garcia (CRAL)
Abstract : In protoplanetary discs, micron-sized grains should grow up to planetesimal sizes in order to ultimately form planets. However, dynamical studies show that once they reach a critical size, they drift rapidly into the accreting star. This is known as the radial drift barrier. Moreover, laboratory experiments have shown that grains can fragment or bounce, stopping the growth towards planetesimal sizes.
In order to overcome those barriers, several methods have been proposed such as particles traps (e.g. vortices or planet gaps) which all involve large-scale dynamics. In this work, we choose to investigate the intrinsic properties of the grains during their growth, in particular their porosity.
We thus consider the growth of grains with variable porosity as a function of their mass in several regimes of compression/expansion (Kataoka et al. 2013, Oku- zumi et al. 2012) and implement it in our 3D SPH two-fluid code (Barrière-Fouchet et al. 2005).
We find that growth is accelerated for porous grains that can reach kilometer sizes. On the other hand, drift is slightly slowed down for porous grains that can grow up to larger sizes before drifting towards the star. As a result, grains in the outer regions of the disc reach larger sizes than when porosity is neglected. Those two mechanisms can help grains overcome the radial-drift barrier and form plane- tesimals. The Stokes drag regime appears to play a substantial part in maintaining grains in the disc.
Considering a constant fragmentation threshold, we also find out that growth is delayed because of fragmentation but reaching large sizes and thus overcoming problems due to fragmentation and radial drift is still possible. However, very fluffy grains are fragile and can be easily disrupted leading to a massive accretion of dust into the star. Moreover, we show that the effects due to dust bouncing can be neglec- ted compared to fragmentation.
Finally, we investigate the influence of the size of monomers and -parameter on the evolution of porosity and then dust in the disc.
Dust growth is accelerated by porosity and thus promotes grains decoupling. Very fluffy grains are more affected by fragmentation. However, dust collective ef- fects and porosity can help grains to overcome planet formation barriers. Besides, the bouncing barrier can be neglected in the case of porous dust compared to other barriers. Finally, the intensity of turbulence can alter the growth and so the outcome of dust. The size of monomers modifies the grain filling factor without impacting dust decoupling in the inner parts of the disc.
Friday 31 August 2018
Title : AstroSat/UVIT deep imaging survey of galaxies in the GOODS-South
Speaker : Kanak Saha (IUCAA)
Abstract :
We present a deep (far and near-) ultra-violet imaging observations of about 600 sq arcmin area of sky in the HST/Chandra Deep Field-South and Beyond, using the Ultra-Violet Imaging Telescope on-board AstroSat. After giving a brief overview of the telescope, current and future prospects of these observations in the context of galaxy formation and cosmology will be discussed. Our preliminary analysis reveals better source statistics compared to the currently available deep UV observation performed by GALEX in the same region of the sky, especially due to a factor of 3 - 4 higher spatial resolution. Improved sensitivity has allowed us to explore detailed physical properties of low-mass star-forming galaxies. In particular, we will discuss a few dwarfs and possible Lyman continuum leaker candidates from our observation.
Friday 27 July 2018
Title : Satellite galaxies and their large-scale environment.
Speaker : Quan Guo (SHAO)
Abstract : In this seminar talk, I willl discuss how we measure the luminosity functions, spatial distribution around isolated galaxies measured from SDSS galaxy samples. And how these properties including angular distribution of satellite galaxies depend on the large scale structure : filaments and how the properties of satellite galaxies in Local Group-like galaxy pair are affected by the large scale environment too.
Friday 20 July 2018
Title : Galaxy Evolution in 3D
Speaker : Lisa Kewley (ANU)
Abstract : Tracing matter and chemical elements in the Universe is critical for understanding the formation of the first galaxies, the formation and growth of super-massive black holes, and ultimately the evolution of galaxies like our Milky Way. Throughout the history of the universe, large-scale gas flows have moulded the arms of spiral galaxies, formed the bulges of the most massive galaxies in the universe, fed supermassive black holes in the centers of galaxies, fueled generation upon generation of new stars, and enriched the intergalactic medium with metals. The physics and impact of these processes can now be traced through high spectral resolution wide integral field spectroscopy.
We use multi-object integral field spectroscopy to build the largest local sample of galaxies with wide 3-dimensional imaging spectroscopy. We combine our local results with insights into the early universe probed through gravitational lensing and adaptive optics. I will present the latest results from our large local and high-redshift 3D surveys to understand the relationship between gas inflows, galactic-scale outflows, star-formation, chemical enrichment, and active galactic nuclei in galaxies. I will finish by discussing how this field will be transformed in Australia through the ASTRO 3D Centre of Excellence as we transition to the era of extremely large ground and space telescopes.
Friday 6 July 2018
Title : On the detection of faint Lyman-alpha haloes in MUSE deep fields.
Speaker : Jean-Baptiste Courbot (INRIA)
Abstract : In this talk I will summarize the works of my PhD that address the detection of faint sources in very noisy data, namely the Lyman alpha light in the MUSE deep field images. I will first present a review of three methods that were developed for this purpose. The first one addresses the problem in an hypothesis-testing framework, which explicits the absence/presence hypothesis. The two latter methods use a Bayesian segmentation framework, assuming a statistical structure in the data. Then, I will present the evaluation on mock data containing faint signals with standard (Gaussian) noise first, and then with randomized MUSE noise. This will highlight the fact that the Bayesian methods are especially robust to very high noise level. Finally results on real MUSE images will be shown, paving the way for large-sample analysis on robustly determined detection maps.
Friday 29 June 2018
Title : Early galaxy evolution : gas and dust in galaxies during the first billion years after the big bang
Speaker : Kristen Knudsen (Onsala Space Observatory)
Abstract : The first billion years after the big bang represents the early parts of galaxy evolution and the epoch of reionization. There has been major progress in finding galaxies at z>6 and now major efforts are done to understand the physical properties of the early galaxies. In this talk I will present some of the recent observational results, in particular with ALMA, and place this in the context of understanding both the early stages of galaxy evolution and what it possibly means for our understanding of the sources that cause the reionization.
Friday 15 June 2018
Title : GalICS 2.1 : disentangling the roles of cold accretion and cooling in the formation of galaxies.
Speaker : Andrea Cattaneo (Paris Observatory)
Abstract : The mass-scale that separates late-type and early-type galaxies is linked to the critical halo mass or the propagation of a stable shock. The critical mass depends on the metallicity of the gas and the shock radius. Dekel & Birnboim (2006) showed that they could reproduce the observed bimodality scale for plausible values of metallicity and shock radius. I shall take their analysis one step further and compute the shock radius from first principles. This advancement allows us to compute an individual shock-heating mass for each halo in our semianalytic model, a new version of which (GalICS 2.1) will be presented in this seminar.
I shall discuss both models in which the shock-heated cools and models with cold-mode accretion only. Without jet heating, cooling overproduces massive galaxies. In conjuction with plausible supernova and quasar feedback, a total shutdown of cooling brings the model in agreement with the galaxy stellar mass function at the low-mass end and at the high-mass end but leads to a dearth of galaxies at intermediate masses. My conclusion is that dwarf and giant galaxies acquired most of their mass through cold flows but cooling is important to form galaxies of intermediate mass, such as Andromeda and the Milky Way.
Friday 8 June 2018
Title : Fours Solaires et Optique Adaptative
Speaker : François Henault (Univ Grenoble)
Abstract : Les méthodes de réglage et de contrôle des miroirs segmentés des grandes installations collectrices d’énergie solaire (de type four solaire ou centrale à tour) présentent quelques liens de parenté avec celles qui sont utilisées aujourd’hui sur les télescopes astronomiques “extrêmement grands” (ELT) et leurs systèmes d’optique adaptative, même si les performances recherchées ne sont pas du même ordre de grandeur. Dans ce séminaire, nous explorerons le domaine des grands observatoires astronomiques terrestres et de l’optique adaptative, en décrivant les principes et les dispositifs instrumentaux utilisés pour mesurer les défauts optiques (senseurs de front d’onde), en mettant en avant quelques analogies avec les installations solaires. Nous verrons ensuite que l’analogie peut être poussée jusqu’aux calculs de flux et de concentration. Par exemple les pertes optiques en concentration solaire peuvent être estimées par un coefficient comparable au “rapport de Strehl” très utilisé en optique adaptative. Je rappellerai enfin les principaux résultats de mon travail de thèse au Four Solaire d’Odeillo (PROMES) sur les méthodes de contrôle des surfaces réflectrices par “rétrovisée”. Il s’agit en fait d’un “senseur de front d’onde solaire” qui est actuellement en cours de développeent par deux étudiants en thèse au PROMES.
Wednesday 6 June 2018
Title : Early Results of the Subaru Hyper Suprime-Cam Survey for High Redshift Galaxies
Speaker : Masami Ouchi (ICRR, Tokyo)
Abstract : I present progresses of our Subaru Hyper Suprime-Cam (HSC) optical imaging survey for high redshift galaxies including Lymanalpha emitters (LAEs), and overview the early results. The data of the early results allow us to obtain over a half million galaxies at z 4-7 in a total area of 100 deg^2 that are about 1-2 orders of magnitude larger than those of previous high-z galaxy surveys. With the large Subaru/HSC galaxy sample supported by the spectra taken with Keck, Subaru, and Magellan, and the large-area deep Spitzer data, I will present various statistical measurements including luminosity functions, correlation functions, rest-frame optical line ratios, and discuss galaxy formation and cosmic reionization based on our initial 12 papers for refereed journals, 10 of which have been, so far, accepted for publication.
Friday 1 June 2018
Title : The inhomogeneous propagation of the Reionization
Speaker : Dominique Aubert (Strasbourg Observatory)
Abstract : The reionization of the Universe is an inhomogeneous process, influenced by the organisation of UV sources and absorbers and their respective evolution. This complexity translates into a non trivial propagation of ionization fronts and growth of HII regions. As a consequence, all galaxies are not reionized at the same time, depending on their environments or buildup histories, whereas ionization fronts can accelerate or slow down depending on local densities.
We investigated this inhomogeneity of the reionization process using cosmological simulations made with the newly developed EMMA AMR code (Aubert et al. 2015). We produced the ‘CoDa-I AMR’ reionization simulation, a 91 cMpc volume sampled with 2048^8 particules/base grid cells, and with enough resolution to track 10^8 solar masses halos. : this simulation models fully coupled radiative-hydrodynamics down to z=6. Halos during the reionization were projected to z=0, using merger trees from a pure dark matter simulation with identical initial conditions. This volume and resolution allowed us to assess both the global reionization of the intergalactic medium and the local reionization times and durations for 20 millions halos at z=0. We find that halos with current masses greater than 10^10 Msol could be reionized 500 millions years before the global IGM, whereas halos less massive than 10^10 Msol reionized at the same time as the IGM or even later. The CoDa-I AMR simulation used CLUES initial conditions and includes a Local Group analog in the proper environment : we find that the Local Group reionized in isolation without influence from nearby clusters and the MW and M31 reionized separately, despite their proximity.
Using another set of EMMA simulations, we also investigated the propagation of ionization fronts during the reionization, by measuring their speeds. We found that front speeds evolve according to a two-stage process : a first slow low-velocity stage, during which fronts have to propagate in dense environments, then a second high-velocity stage, during which front speeds get close to the speed of light as they progress in the more diffuse IGM before the overlap of HII regions. We also assessed the influence of reduced speed of light approximations in this evolution and how it can affect simulation predictions.
Friday 25 May 2018
Title : Making it up to E(A)LT’s : extremely (adaptive) large telescopes
Speaker : Carlos Correia (LAM)
Abstract : Observations with giant segmented groundtelescopes promise to revolutionise VIS and NIR astronomy. Yet, to take full benefit of the collecting power, resolution (and sensitivity) of these huge machines, adaptive optics systems will become mainstream and are to assist 100% of the observations, as ELTs are too large to operate as conventional seeing-limited telescopes.
I will provide an overview of current developments on novel wave-front sensing techniques using natural and laser guide stars, advanced wave-front reconstruction for the different AO modes and the challenges faced when customising AO post-processing in order to meet stringent science goals. As we go, I will show how these developments are propelling the design of Harmoni, the European Extremely Large Telescope’s 1st light IFU.
Friday 18 May 2018
Title : Non-linear reconstruction of cosmological density and velocity fields for massive spectroscopic surveys via Fast Action Minimization method.
Speaker : Elena Sarpa (LAM)
Abstract : Cosmological massive spectroscopic surveys benefit from an accurate modelling of the density and velocity fields, ideally down to the high-density regions in which the gravitational clustering attains the non-linear regime. This can be achieved by so-called reconstruction techniques, modelling backward-in-time the trajectories of galaxies, or any point-like particles interacting only by gravity in an expanding Universe. Nowadays those techniques are regularly used to improve the analysis of Barionic Acoustic Oscillations (BAO), providing an useful tool for non-parametric modelling of Redshift Space Distortion (RSD). We are developing a new version of the Fast Action Minimization method (FAM, Nusser & Branchini 2000), dubbed extended-FAM or eFAM, intended to reconstruct the trajectories of more than 10^5 particles in a generic cosmology. Particular attention is dedicated to the application of the eFAM to Nbody simulations in order to show its potentiality in improving the measurement of the BAO peak by reconstruction of the density field back to an epoch prior non-linear evolution. Moreover we’ll present the ability of eFAM in recovering the velocity field.
Friday 4 May 2018
Title : Galaxy kinematics in cosmological simulations : connecting angular momentum, spin, mass growth and environment
Speaker : Claudia Lagos (ICRAR, Perth)
Abstract : Measurements of the angular momentum and spin of galaxies are becoming widely available thanks to the new generation of extragalactic IFU surveys. This is opening a new window in which to investigate galaxies and specifically the connection between their mass growth, quenching, morphological and kinematic transformation. Simulations of galaxy formation provide us with a unique opportunity to study causality in all the emerging correlations, such as the specific angular momentum-mass, spin-ellipticity, among other relations. We use the EAGLE and Hydrangea hydrodynamical simulation suites to investigate (i) the specific angular momentum of galaxies and how it traces the formation history of galaxies, (ii) quantify the effect of galaxy mergers of different types on the kinematics of galaxies, and (iii) distinguish nature vs. nurture in the spin evolution of galaxies. We place our results in the context of the latest IFU observational results at low and high redshift.
Friday 27 April 2018
Title : Large-scale structure of the Universe : observer’s point of view
Speaker : Elmo Tempel (AIP)
Abstract : The cosmic web is a highly complex geometrical pattern, with galaxy clusters at the intersection of filaments and filaments at the intersection of walls. Using observational data, we can visually recognize the main components of the cosmic web : voids, filaments and (super)clusters. However, to classify the cosmic web using mathematical methods is much more complicated task, which also involves the analysis of observational selection effects. In my talk I will give a brief overview about the observed large-scale structure together with the main selection effects that should be taken into account while analyzing the data.
Half of my talk is dedicated to galactic filaments in the cosmic web. We have developed a probabilistic filament finder, called Bisous model. The Bisous filament finder works directly with the galaxy distribution data and the model intrinsically takes into account the connectivity of the filamentary network. The model is applied to various observed and simulated datasets. Several applications of the Bisous model will be introduced during the talk.
Friday 20 April 2018
Title : Unveiling the cosmic web with Lyα emission from IGM filaments
Speaker : Marta Silva (University of Groningen)
Abstract : The IGM filamentary structure contains a large fraction of the baryons in the Universe. This important component of our Universe is formed due to gravitational collapse, as a result, the statistics of large-scale structures distribution can be determined by the cosmological setup of our Universe. Observationally, large-scale IGM filaments are mostly indirectly detected through absorption of radiation from a background source or inferred from galaxy positions. I will discuss the prospects to directly detect these filaments through their Lyman-alpha emission. Using the Eagle simulation particle data we made predictions for Lyman alpha IGM emission. We find that inside the densest filaments the Lyman alpha emission signal can reach values of the order of 10^-20 erg s^-1 cm-2 arcsec^-2 at z=4. Although, this signal can decrease by a few orders of magnitude depending on the target position and on whether the gas is in ionization equilibrium. The uncertainty in the signal is dominated by the strength and fluctuations of the UV background and by the clumping of the gas in IGM filaments. These two quantities are key parameters to the determination of the ionization and thermal state of the gas in IGM filaments and will, therefore, have a meaningful impact on the Lyman alpha emission intensity due to recombinations, collisions, and scattering of high order Lyman series photons.
Friday 6 April 2018
Title : Taking a modern look at old friends : MUSE observations of oxygen-rich supernova remnants
Speaker : Frédéric Vogt (ESO)
Abstract : Oxygen-rich (O-rich) core-collapse supernova remnants (SNRs) provide us with excellent means to study the formation of heavy elements inside massive stars, and the physics of supernova explosions. In these SNRs, fast moving stellar ejecta shine brightly in forbidden oxygen lines such as [OIII] 5007Å, and are largely devoided of hydrogen emission, indicating that the progenitor must have shed a large fraction of its outer envelope prior to the SN explosion. O-rich SNRs are also young, so that interactions with the surrounding medium have not (yet) altered the kinematics or composition of the ejecta significantly. In essence, O-rich SNRs provide us with an exploded view -literally !- of a massive star that reached the end of the Silicon burning sequence.
In this talk, I will present our MUSE observations of O-rich SNR 1E0102.2-7219 in the SMC. The datacube revealed a treasure trove of anticipated and unanticipated scientific information, which ultimately led to the first identification of the compact object in archival Chandra observations of this SNR : an isolated neutron star, akin to those found in Cas A and Puppis A, and the first extragalactic member of its kind. I will also discussed ongoing MUSE observational efforts built upon the legacy of 1E0102.2-7219, and aimed at assembling the most detailed view of a recently-dead massive star to-date.
Friday 16 March 2018
Title : The Sphinx simulations : the first billion years and cosmic reionisation
Speaker : Joakim Rosdahl (CRAL)
Abstract : The epoch of reionisation marks a major shift from a cold neutral Universe to a warm ionised one, a transition which was thought to be powered by UV radiation emitted from young massive stars in the first galaxies.
Our understanding of this epoch is still limited : observationally we glimpse a handful of the most luminous galaxies existing at the end of the epoch, but with the advent of the James Webb Telescope and other upcoming instruments we will soon start getting better information about the sources powering reionisation. Theoretically, the best way to gain an understanding is with cosmological simulations. However, those are very expensive, so either people simulate large patches of the reionising Universe without resolving the galactic sources, or go for tiny patches where the stellar sources inside one or a few galaxies are resolved, but the large-scale process is lost. I will present a new suite of simulations, called the Sphinx project, where we have developed new methods that allow us to perform radiation-hydrodynamical simulations of cosmic reionisation, resolving the emission and escape of radiation in the inter-stellar medium of hundreds of galaxies all evolving together in the same simulation, hence capturing the interplay of small- and large-scale processes.
I will describe the simulations and the key developments that made them possible, and show results from a pilot Sphinx study looking into the surprising impact on reionisation of spectral energy distribution (SED) models containing binary stellar populations, compared to more classical SEDs.
Friday 9 March 2018
Title : How to measure gas accretion on halo scales - A MEGAFLOW accretion study
Speaker : Johannes Zabl (IRAP - Toulouse)
Abstract : In recent years a strong body of observational evidence has emerged suggesting that galaxies need to accrete gas from their surrounding medium. While the global evidence seems conclusive, one of the main challenges of contemporary astronomy is to directly observe this gas accretion around individual galaxies. Background quasars are rare and thanks to the MusE GAs FLOw and Wind [MEGAFLOW] survey, which is targeting 80+ MgII systems at z=0.3-1.4, it is possible to have statistical samples of galaxy-absorber pairs to constrain the kinematics of the circumgalactic gas, both for outflowing and inflowing gas.
In this talk, I will focus on those galaxy-absorber pairs that we can identify as accretion cases. I will present the methodology to test the existence of predicted cold gas accretion-disks that are co-aligned and co-rotating with the associated galaxies, demonstrate how we infer an approximate accretion rate, and discuss possibilities for future collaborations with experts at CRAL.
Friday 2 March 2018
Title : Orbital characterization of giant exoplanets and brown dwarfs with the VLT planet finder SPHERE
Speaker : Anne-Lise Maire (MPIA)
Abstract : Astrometric monitoring of directly imaged exoplanets and brown dwarfs allows the study of their orbital parameters and system architectures. Because of the long orbital periods and/or faintness of these objects, accurate astrometry is challenging when based on data acquired on timescales of a few years. The new high-contrast imaging instrument VLT/SPHERE provides exquisite contrasts and astrometric accuracies down to about 1 mas thanks to its dedicated design and the use of optimized observational strategy. We will present the results of the astrometric monitorings of the brown dwarf HR2562 B and giant exoplanet 51 Eridani b monitored as part of the SPHERE consortium exoplanet imaging survey.
Friday 16 February 2018
Title : Star-forming environments throughout the M101 Group
Speaker : Aaron Watkins (Oulu U.)
Abstract : I present the results of deep narrow-band imaging done with the Burrell Schmidt Telescope at Kitt Peak National Observatory, targeting H alpha emission in the nearby spiral M101 and in its lower mass companions. In conjunction with deep archival GALEX imaging, I compare the H alpha/FUV flux ratios—a tracer of the IMF in young populations—of HII regions found in three different star-forming environments : M101’s inner disk, its faint outer disk, and its low-mass companion NGC 5474. I find that, once bulk radial trends in extinction are taken into account, the median value of and dispersion in H-alpha/FUV is invariant across these environments. Through comparisons with Starburst99 models, I show that these distributions of H-alpha/FUV, including those found in very low-density and low-SFR environments, can be explained without resorting to a mass truncation in the IMF.
Friday 09 February 2018
Title : Noise-based detection paradigm, Gnuastro and reproducible science
Speaker : Mohammad Akhlaghi (CRAL)
Abstract : Astronomical instrumentation has greatly advanced over the last 40 years : with digital detectors, space telescopes and +8m class ground-based telescopes for example. However, the signal-based detection paradigm (for example from Petrosian or Kron in the 1970s) is still the primary method of low-level data analysis (detection). In this talk, I will introduce a fundamentally new noise-based detection paradigm for detecting signal with an extremely low signal-to-noise ratio. With thresholds that are far below the sky value and non-parametric expansion into noise, it is successfully able to detect very diffuse and irregularly signal in noise. The software implementation is called NoiseChisel. Even though it was initially developed for galaxy evolution studies, it can be used in any other contexts where the desired signal can be diffuse and irregularly shaped. Since coming to CRAL, it has been extended to 3D datasets like those of MUSE. The talk will continue with an introduction to NoiseChisel’s parent software : GNU Astronomy Utilities or Gnuastro. It is a large package of useful programs and libraries for astronomical data analysis directly on the command-line without the need to use mini-environments like IRAF or Python. It fully conforms with the GNU Coding Standards for easy integration into all Unix-like operating systems and familiar installation, usage and documentation. The talk will conclude with a discussion on reproducible scientific papers and a solution that has already been implemented to have exactly reproducible scientific papers. My next CRAL Seminar will focus on the scientific results obtained with these low-level tools.
In the afternoon, a tutorial session will be held to discuss best practices in using Gnuastro to detect objects successfully using NoiseChisel. Everyone is welcome to join the tutorial and bring their own images to test it on their own systems (the code is publicly released, first link below). But Gnuastro isn’t limited to detection in images, if you work with FITS files (cubes, images or tables), you will find many convenient programs in Gnuastro that will make working with FITS files (keywords, extensions, images or tables) as easy as the "ls" and "cp" commands you use every day. The tutorial will follow the second link below as a reference.
https://www.gnu.org/software/gnuastro/
https://www.gnu.org/software/gnuastro/manual/html_node/General-program-usage-tutorial.html
Friday 26 January 2018
Title : Cosmological mass function
Speaker : Jan J. Ostrowski (CRAL)
Abstract : Realistic description of cosmological structure formation is an important challenge from both theoretical and numerical point of views. In my presentation I will give a brief prescription for a semi-analytic treatment of structure formation and a resulting mass function on galaxy cluster scales in a highly generic scenario. This will be obtained by an exact scalar averaging scheme together with the relativistic generalization of Zel’dovich’s approximation (RZA) that serves as a closure condition for the averaged equations. Some results related to the ’silent universe’ model will be also presented.
Friday 19 January 2018
Title : Methods for analyzing MUSE extragalactic deep-fields
Speaker : Raphaël Bacher (Univ. Grenoble)
Abstract : MUSE is a new European instrument able to observe the Universe and its first galaxies in more than 3600 different colors. The study of these galaxies colors allows the analysis of their chemical and physical properties. The first part of this talk consists in estimating the spectrum of each galaxy, despite perturbations from the atmosphere. To perform this estimation, data fusion and spectral unmixing approaches are developed, using complementary data from the Hubble Space Telescope. The second goal of this work is the detection of the Circum-Galactic Medium (CGM), that is a cloud of gas surrounding some galaxies. This CGM is characterized by a faint spatially extended signature. To detect this signature, new detection methods are proposed, ensuring a robust control of detection errors.
Friday 12 January 2018
Title : Machine Learning Applications for astronomy.
Speaker : Pierre-Yves Lablanche (AIMS - South Africa)
Abstract : Astronomy and astrophysics like many other scientific field is entering the big data era. Observational instruments such as the Large Synoptic Survey Telescope (LSST) and the Square Kilometre Array (SKA) will produce hundreds of petabytes of data per year supporting many scientific projects and existing analysis and processing tools will not be sufficient to tackle the upcoming “big data” challenge. As a consequence a lot of focus has been given to machine learning techniques both supervised and unsupervised over the past years. In this talk I will present a case study of galaxies identification using unsupervised learning and discuss the results evaluation as well as the method application to future astronomical data. In addition I will give insights about some of the latest machine learning technique being used and developed within the astronomical community.
Friday 15 December 2017
Title : A holistic approach for cosmology with extragalactic surveys
Speaker : Johan Comparat (MPE)
Abstract : Physical cosmology studies our Universe, its origin, evolution and the laws governing it. In the last decades a concordance cosmological model and the concept of precision cosmology emerged. The concordance model successfully explains the most accurate cosmological measurements available to date. Precision cosmology now seeks after shrinking all possible measurement error. The forward model approach is the concept of modeling the complete measurement process down to its tiniest details using a state-of-the-art theoretical framework. To enhance the precision of our understanding of the Universe, we thus need to apply forward models on the largest possible data sets. In the coming decade, extragalactic surveys are about to overcome statistical errors with data sets a hundred times larger than today’s. Forward modeling is the key to more robust and more accurate cosmological statements based on extragalactic surveys. In this presentation I will first detail the upcoming challenge to forward model upcoming massive spectroscopic surveys e.g. 4MOST, DESI, MSE ... Then I illustrate this challenge based on recent results of the SDSS-IV eBOSS survey, a pathfinder for these next generation surveys.
Friday 1 December 2017
Title : E=MC2 and the negative pressure of dark energy
Speaker : Bernard Castaing (LEGI Grenoble)
Abstract : (Common seminar with ENS de Lyon Physics) According to recent observations, the Dark Energy would represent 70% of the content of our Universe. The most popular way to account for this Dark Energy make use of the Cosmological Constant introduced by Einstein. However, some uncertainty remain about the theoretical value this constant should have. Alternate approaches have to introduce a fluid whose pressure would be negative. In this paper, we present unusual particles which could form a gaz with positive thermodynamical pressure, while presenting all the gravitational characteristics of Dark Energy.
Friday 10 November 2017
Title : Getting ready for JWST with new-generation spectral models and interpretation tools
Speaker : Jacopo Chevallard (IAP)
Abstract : The James Webb Space Telescope (JWST), scheduled for launch in October 2018, will open a largely unexplored observational window at near- to mid-infrared wavelengths (0.6-28 micron). In particular, the Near-InfraRed Spectrograph (NIRSpec) on board JWST will allow the detection of standard UV and optical emission lines out to redshift 10 and beyond for large samples of galaxies. This will provide unique insight into, for example, the star formation activity and early chemical enrichment of galaxies, the co-evolution of galaxies and AGNs, the impact of stellar and AGN feedback, and the main drivers of cosmic reionization. One of the first large programs to exploit the new observational capabilities of JWST will be the NIRSpec Guaranteed-Time-Observations (GTO) program, which includes a multi-layered survey in Multi-Object-Spectroscopy (MOS) mode targeting 104 galaxies at 1.5 < z < 10, as well as an Integral-Field-Unit (IFU) survey targeting 60 galaxies and AGNs out to z 9.
In this talk, after a short review of JWST and the NIRSpec GTO program, I will present a set of physically-motivated simulations of NIRSpec and NIRCam (the Near-Infrared Camera on board JWST) observations, which we have used to optimise the survey design, and which will be made available to the community on time for the preparation of Cycle-1 General-Observer (GO) observations (call for proposals on 30/11/2017). I will also discuss a new set of state-of-the-art population synthesis+photoionization models, developed in collaboration with the NEOGAL team at the IAP, tailored to the interpretation of high-redshift galaxy observations. I will show how these models, incorporated into our new-generation analysis tool "Beagle", successfully reproduce a broad range of UV and optical features from high-quality spectra of nearby galaxies, as well as the high-ionization UV lines observed in galaxies at z > 6. The ability to self-consistently measure the physical properties of gas and stellar populations across a broad range of redshifts and stellar masses will be crucial to maximise the scientific return of JWST and gain unique insight into the evolution of galaxies across cosmic times.
Friday 3 November 2017
Title : Exploring the Universe with Quasar Absorption Spectra : correlations among tracers of the mass density field and the impact of ionizing background intensity fluctuations
Speaker : Satya Gontcho A Gontcho (U Barcelona).
Abstract : The work I present revolves around the use of quasar spectra to probe the Universe. Absorption features in the electromagnetic spectrum of bright distant quasars by ionized hydrogen give us access to the distribution of gas in the Universe. Because the light from distant quasars is redshifted as it travels through the Universe, photons emitted with an energy above the Lyman-alpha transition will be absorbed along their journey, at the point where their wavelength has been redshifted to the transition wavelength of 1216 Angstrom. As a result, an observed spectrum and its features, referred to as Lyman-alpha Forest, depend on the density of neutral hydrogen as a function of position along the line of sight from the quasar. Similar studies can be made with photons blueward of the emission line of the triply ionized carbon. Studying the distribution of this ionized gas on cosmological scales allows us to trace the underlying matter density field while at the same time gaining insight on the thermal state of the intergalactic medium and its enrichment processes. We also present an analytical framework to understand the effects of a fluctuating intensity of the cosmic ionizing background on the correlations of the Lyα Forest transmission fraction measured in quasar spectra.
Friday 20 October 2017
Title : The Faint End of the HI Mass Function.
Speaker : John Cannon (Macalester College)
Abstract : Low-mass galaxies are the most numerous type of extragalactic system at all epochs of the universe. The population of low-mass galaxies in the local volume allows unique astrophysical and cosmological perspectives that are unavailable in more distant or more massive systems. The ALFALFA blind extragalactic HI survey has cataloged tens of thousands of gas-rich galaxies in the local universe and has populated the faint end of the HI mass function with statistical confidence for the first time. In this talk I will present results from comprehensive follow-up observing campaigns designed to study the low-redshift, low-mass, gas-rich population discovered by ALFALFA. The centerpiece of this effort is the Survey of HI in Extremely Low-mass Dwarfs (SHIELD), an ongoing multi-wavelength investigation of the properties of 82 extremely low-mass galaxies selected from the complete ALFALFA catalog. I will also discuss results from parallel ongoing observing programs that explore more exotic low-mass objects, including "ultra compact high velocity clouds" (HI clouds with structural parameters that match those of gas-bearing "mini-halos" if located within 1 Mpc), candidate "dark galaxies" (systems with extreme hydrogen mass to stellar light ratios), and targeted studies of individual sources of interest (including two of the most metal-poor galaxies known in the universe). Taken as a whole, these observing programs are furthering our understanding of the continuum of galaxy properties in the low-mass regime.
Friday 30 June 2017
Title : Statistical isotropy of the Cosmos from Planck.
Speaker : Tarun Souradeep (IUCAA)
Abstract : The widely heralded and remarkable progress in cosmology leading to the emergence of a ’standard cosmological model’ has relied on certain key assumptions at various levels. I briefly review work along multiple facets, largely, from the research program of my group at IUCAA, that have all attempted to adopt an agnostic approach in drawing cosmological inference independent of such assumptions, in the context of the exquisite observations of the Cosmic Microwave Background CMB anisotropy by Planck. I dwell on the enigmatic Cosmic Hemispherical Anisotropy that continues to challenge our understanding of the Universe.
Friday 23 June 2017
Title : The Square Kilometre Array
Speaker : Jeff Wagg (SKA)
Abstract : Building on the major scientific achievements of the current generation of centimetre and metre-wavelength telescopes, phase 1 of the Square Kilometre Array (SKA1) will be the next global radio astronomy observatory. It is currently being designed by engineers and scientists from around the world, with the aim of beginning full science operations in the 2020s. SKA1 will conduct key tests of general relativity through surveys and timing of pulsars in our Galaxy. It will help answer fundamental questions related to the evolution of atomic Hydrogen in the Universe, from the present day back to the Cosmic Dawn. I will give an overview of the wide breadth of science objectives that could be addressed with SKA1, along with a description of the planned telescopes and the sites that will host them.
Friday 9 June 2017
Title : The self induced secular evolution of gravitating systems
Speaker : Christophe Pichon (IAP)
Abstract : Since the seminal work of Einstein, physicists have understood in the context of kinetic theory how ink slowly diffuses in a glass of water. The fluctuations of the stochastic forces acting on water molecules drive the diffusion of the ink in the fluid. This is the archetype of a process described by the so-called fluctuation-dissipation theorem, which universally relates the rate of diffusion to the power spectrum of the fluctuating forces. For stars in galaxies, a similar process occurs but with two significant differences, due to the long-range nature of the gravitational interaction : (i) for the diffusion to be effective, stars need to resonate, i.e. present commensurable frequencies, otherwise they only follow the orbit imposed by their mean field ; (ii) the amplitudes of the induced fluctuating forces are significantly boosted by collective effects, i.e. by the fact that, because of self-gravity, each star generates a wake in its neighbours. In the expanding universe, an overdense perturbation passing a critical threshold will collapse onto itself and, through violent relaxation and mergers, rapidly converge towards a stationary, phase-mixed and highly symmetric state, with a partially frozen orbital structure. The object is then locked in a quasi-stationary state imposed by its mean gravitational field. Of particular interests are strongly responsive colder systems which, given time and kicks, find the opportunity to significantly reshuffle their orbital structure towards more likely configurations. This presentation aims to explain this long-term reshuffling called gravity-driven secular evolution on cosmic timescales, described by extended kinetic theory. I will illustrate this with radial migration, disc thickening and the stellar cluster in the galactic centre.
Friday 2 June 2017
Title : Dusty spirals triggered by shadows in protoplanetary disks
Speaker : Nicolás Cuello (Pontifica University)
Abstract : Spirals waves form in protoplanetary disks (PPDs) around young stars due to different physical mechanisms : planet torques, gravitational perturbations and illumination effects. Although recent near-infrared and sub-millimetric observations revealed astonishing spiral-shaped features in PPDs, there is an active discussion to understand how they formed. Regardless of their origin, spirals are characterized by a local increase in pressure, which translates into radial and azimuthal asymmetries in the gas and in the dust distribution. In this work, we focus on the spirals formed in the gaseous phase due to shadows cast at fixed disk locations (Montesinos et al. 2016). In particular, we study their effect on the dynamics of the dust particles in PPDs. Our main finding is that dust particles of different sizes and densities are efficiently trapped inside these shadow-triggered spirals. We also perform radiative transfer calculations to model the thermal emission of the dust, which reveal that the resulting dust structures are in good agreement with the recent observations. Finally, we emulate millimetric-wavelength observations and find that these dusty spirals are detectable with ALMA in principle. Interestingly, the detection of the spirals depends on the size of the inner disk cavity. Given that these dusty features lower the relative velocities among solids, they constitute "sweet spots" for dust growth and result in an efficient mixing of different dust species. These aspects will be discussed in the context of planet formation.
Friday 19 May 2017
Title : Etats des lieux de la région transneptunienne avant l’ère JWST
Speaker : Aurélie Guilbert-Lepoutre (UTINAM)
Abstract : Les objets transneptuniens (OTNs) sont les parents de nombreuses populations de petits corps dans le système solaire : comètes, Centaures, et sans doute aussi une partie des astéroïdes. Au moment de leur découverte, ils ont fourni des diagnostiques qui ont permis de revoir complètement notre paradigme du système solaire, passant d’une vision très statique à une vision selon laquelle les planètes auraient migré pour former les structures dynamiques observées aujourd’hui. Pourtant, étant les objets les plus distants, étudier leurs propriétés physiques (qui permettraient de confirmer les scenarii d’évolution du système solaire) et leur lien avec les autres populations de petits corps est encore aujourd’hui extrêmement difficile. Les études photométriques et spectroscopiques effectuées depuis le sol et l’espace ont de plus dressé un tableau bien difficile à interpréter. Au cours de ma présentation, j’établierai un état des lieux de nos connaissances actuelles sur la région transneptunienne, en lien avec les résultats de missions spatiales telles que NASA/New Horizons et ESA/Rosetta, ainsi que des grandes avancées que nous espérons faire avec le futur télescope spatial JWST.
Friday 12 May 2017
Title : Dark energy without dark energy : Observational tests and theoretical challenges
Speaker : David L. Wiltshire (University of Canterbury)
Abstract : I will give an overview of the timescape cosmology. It is assumed that inhomogeneities - voids, walls and filaments - modify the average background geometry of the universe, which is no longer a simple solution of Einstein’s equations with homogeneous dust. To obtain a viable phenomenology without dark energy, I provide a framework for interpreting Buchert’s backreaction formalism, by revisiting fundamental issues relating to the definition of gravitational energy in a complex geometry.
Cosmic acceleration is realized as an apparent effect due both to backreaction and the calibration of the clocks of observers in gravitationally bound structures relative to the time parameter that best describes the average statistical evolution. The cosmic coincidence problem is solved directly in relation to the growth of the void fraction.
Predictions of the timescape phenomenology are very close to the standard cosmology, but with differences which can be tested. I will outline current observational constraints, future tests (e.g., with the Euclid satellite), and also theoretical challenges that need to be overcome for backreaction models to fully compete with the Lambda Cold Dark Matter cosmology.
Friday 05 May 2017
Title : Planetesimal Formation through the Streaming Instability
Speaker : Chao-Chin Yang (Lund Observatory)
Abstract : The formation of kilometer-scale planetesimals is an important stage in the course of planet formation around young stars. It is faced with several major barriers, though. Direct dust growth by coagulation is limited, up to mm to cm in size, due to inefficient sticking, bouncing, and fragmentation at collision. Even if the solid particles manage to grow past cm in size, they continually lose angular momentum to their surrounding gas due to constant head wind, leading to rapid orbital decay to the star. One promising mechanism for circumventing these barriers is the streaming instability, in which the solids actively participate in the particle-gas dynamics to concentrate themselves to high density, leading to direct gravitational collapse and the formation of planetesimals.
I will review our current understanding of the streaming instability and planetesimal formation. Specifically, how the instability operates, under what conditions it drives strong concentration, the initial mass function of the resulting planetesimals, and its possible interaction with turbulent gas will be examined. Unexplored topics that require further study will also be discussed.
Friday 14 April 2017
Title : Dwarf Galaxies at the Peak Epoch of Star Formation
Speaker : Brian Siana (UC Riverside)
Abstract : Low mass galaxies become more and more prevalent in the early universe, and are thought to be responsible for a number of important phenomena (e.g. reionization and enrichment of the intergalactic medium). I will present our work determining the number density of low mass (6 < log(M*) < 9) galaxies at 1 < z < 3 behind strong lensing clusters using deep near-ultraviolet imaging with the Hubble Space Telescope. I will finish by discussing a number of current investigations into the dust properties, star formation histories, and Lyman continuum escape fractions of these galaxies to foster further discussion during my stay at the Observatory.
Friday 07 April 2017
Title : Dust evolution in circumstellar disks
Speaker : Joanna Drążkowska (University of Zurich)
Abstract : There are more than 3000 known exoplanets and statistical analysis show that every star in our Galaxy should have at least one planet on average. However, the process of planet formation remains poorly understood. Early stages of this process can be constrained thanks to the interferometric observations of circumstellar disks and there is evidence that they have to do with evolution of small solid grains : the protoplanetary dust. In this talk, will give a broad introduction to the protoplanetary dust evolution field, including key physical processes, possible modelling techniques as well as the growth barriers problem. I will also discuss possible scenarios of overcoming the growth barriers and forming planetesimals and planets. I will present my recent work on planetesimal formation in so-called pebble pile-ups and highlight importance of the water snow-line in triggering planet formation.
Wednesday 22 March 2017
Title : The Evanescent wave coronagraph : Principle, Theoretical performance and preliminary results
Speaker : Christophe Buisset (NARIT, Thailand)
Abstract : NARIT is a leading organization in Thailand and South-East Asia based in Chiang Mai. In particular, NARIT manages the Thai National Observatory located at a 2,457 m site on Doi Inthanon, with the new 2.4-meter Thai National Telescope (TNT) which is currently the largest in South East Asia. NARIT also comprises an optical laboratory which includes all the essential equipment to perform cutting edge research in optics. During this seminar, I will describe and present the preliminary the results obtained on the so-called “Evanescent wave coronagraph” (EvaWaCo). This coronagraph is a “band-limited” Lyot coronagraph which mask involves the frustrated total internal reflection phenomenon to produce the coronographic effect. The first part will be dedicated to the description of this coronagraph and to the calculation of the mask transmission. In particular, I will demonstrate that this mask transmission adapts itself to wavelength due to the evanescent wave properties. The second part will be focused on the results of the experimental demonstration of the principle. I will describe the setup we have used and the results we have obtained. The third part of this talk will be focused on the future development of this setup : installation of apodization mask to improve the rejection in the Airy ring area.
Friday 17 March 2017
Title : The Life and Death of Star-Forming Molecular Clouds.
Speaker : Sam Geen (University of Heidelberg)
Abstract : Stars are born from dense gas in molecular clouds. These stars produce energy in radiation, winds and supernovae, which drive outflows and reduce or stop additional star formation in the cloud. In this talk I will present our recent results from radiative MHD simulations of star-forming clouds. In particular I will focus on the effectiveness of each of the stellar processes in driving outflows in a cloud environment, and on how observations of local star-forming clouds can be interpreted in comparison to simulations.
Friday 10 March 2017
Title : Cosmology with large spectroscopy surveys
Speaker : Anand Raichoor (EPFL)
Abstract : Cosmology has now entered a new era with the advent of large, international experiments to constrain the dark energy. Baryon Acoustic Oscillation (BAO), one of the main cosmological probe for dark energy, requires the spectroscopic observation of a large number (10^5-10^7) of galaxies. I will present the SDSS/eBOSS survey, the currently largest on going BAO experiment, with focusing on the z 0.8 tracer, the Emission Line Galaxies (ELGs). I will then introduce the 4MOST/Cosmology survey, 4MOST being a major ESO massive spectroscopic survey. Additionnaly to BAO measurements, the 4MOST/Cosmology survey will enable key cosmological projects, with cross-correlating with other experiments, such as weak lensing ones.
Friday 24 February 2017
Title : The MESSIER surveyor : lifting the veil on the ultra-low surface brightness universe
Speaker : David Valls-Gabaud (Paris Observatory)
Abstract : The S-class MESSIER satellite has been designed to explore the extremely low surface brightness universe at UV and optical wavelengths. The two driving science cases target the mildly- and highly non-linear regimes of structure formation to test two key predictions of the LCDM scenario : (1) the detection of the putative large number of galaxy satellites, and (2) the identification of the filaments of the cosmic web. The satellite will drift scan the entire sky in 6 bands covering the 200-1000 nm wavelength range to reach the unprecedented surface brightness levels of 34 mag/arcsec^2 in the optical and 37 mag/arcsec^2 in the UV. Many important secondary science cases will result as free by-products and will be discussed in some detail, such as the luminosity function of galaxies, the contribution and role of intracluster light, the cosmological background radiation at UV and optical wavelengths, the molecular hydrogen content of galaxies at z=0.25, time domain studies of supernovae, GRBs and tidal disruption events, the chemical enrichment of the interstellar medium through mass loss of red giant stars and the accurate measure of the BAO scale at z=0.7 with over 30 million galaxies detected in Lyman-alpha at this redshift. It will provide the astronomical community the first space-based reference UV-optical photometric catalogue of the entire sky, and synergies with GAIA, EUCLID and WFIRST will also be discussed. Technical issues will likewise be addressed for possible improvements on the current design.
Friday 20 January 2017
Title : Science in the SKA era
Speaker : Mamta Pommier (CRAL)
Abstract : The Square Kilometre Array (SKA) is the 21st century world’s largest radio telescope which will answer fundamental questions about the origin and evolution of the Universe. It is an aperture synthesis telescope with a collecting area of up to one million square metres spread over at least 3000 km, providing a sensitivity 50 times higher than the Expanded Very Large Array and an instantaneous FOV of several 10s of degrees and above. Five key science projects for the SKA have been identified, focused on unsolved problems in fundamental physics, cosmology, galaxy evolution, planet formation and astrobiology. In addition the vast increase in sensitivity provided by the SKA will also almost certainly lead to the discovery of new and totally unexpected celestial phenomena. In this talk, I will describe the main features of the SKA, outline plans for the final design and phased implementation of the telescope, On-going key projects with the SKA path finders/precursors around the world and highlight new results on extra galactic projects (Clusters, galaxies).
Friday 16 December 2016
Title : Kinematics of COSMOS star-forming galaxies over the last billion years
Speaker : Debora Pelliccia (LAM, Marseille)
Abstract : In the local Universe it exists a tight relation between galaxy morphology and other physical parameters, like the galaxy internal kinematics, but at higher redshift it is not clear if this relation still exists. The galaxy kinematics is one of the best tool to study the physical processes that govern the galaxy formation, since it traces the galaxy internal distributions of luminous and dark matter and their evolution with time. I will present our survey HR-COSMOS aimed to obtain the first statistical and representative sample to study the kinematics of star-forming galaxies in the treasury HST/ACS COSMOS deep field at redshifts between 0 and 1.2. About 800 emission-line galaxies were observed with the multi-slit spectrograph ESO-VLT/VIMOS in high-resolution spectral mode (R=2500). I performed a kinematic analysis of the sub-sample of 82 galaxies in the highest redshift range, 0.75 and 1.2, and I established the stellar-mass Tully Fisher scaling relation at z approximately 0.9. Using the same methodology and data analysis, I analyzed the sub-sample of 186 galaxies at the lowest redshift range 0.01 and 0.375 and I confirmed beyond a doubt the non evolution of the stellar mass Tully-Fisher relation since the last 8 billion year. I have moreover derived the dynamical masses of those two sub-samples and investigated the comparison with the stellar masses as function of time and galaxy stellar mass.
Friday 09 December 2016
Title : Spectral Energy Distribution of Galaxies : a tool to probe rapid star formation quenching.
Speaker : Laure Ciesla (CEA, Paris)
Abstract : With the wealth of ancillary data available in large domains of the spectrum (GALEX, HST, CFHT, Spitzer, Herschel, etc), panchromatic studies of galaxies are thus one of the keys to understand how galaxies evolved since their formation. After describing the assets of spectral energy distributions (SED) analysis and explaining how we can model them, I will show with a concrete case how we can identify and characterize galaxies than underwent a rapid star formation quenching with a sample of well-known local galaxies, the Herschel Reference Survey. This sample contains galaxies from the field but also from the dense environment of the Virgo cluster. From this pilot study, I will go to higher redshifts in order to blindly identify sources that have just been quenched using CANDELS/GOODS-South data and try to identify possible causes for this.
Friday 02 December 2016
Title : Hubble Frontiers Fields : A new Era for Gravitational Lensing & Cosmology
Speaker : Mathilde Jauzac (Durham University)
Abstract : The Hubble Frontier Fields (HFF) initiative constitutes the largest commitment ever of HST time to the exploration of the distant Universe via gravitational lensing by massive galaxy clusters. This program devotes 140 orbits of HST time to deep imaging observations of each of six cluster lenses reaching m 29 (AB) uniformly from the optical to the near-infrared. These clusters were chosen for their strong lens properties, and are all highly disturbed objects, showing major and minor merging on-going processes, making them ideal targets to trace the Cosmic Web assembly. While combining strong and weak-lensing regimes to map the total mass with X-rays observations of the hot gas and spectroscopy of cluster galaxies to look at their direction of motion, we can thus study the dynamical scenarios in place within these massive galaxy clusters, and trace the sub-structures engaged. I will present a new multi-wavelength picture of the first two HFF clusters. The depth of these dataset makes these clusters amazing Cosmic Telescopes, but also enables us to get an unprecedented understanding of the cluster physics. I will present a comparison of the dark matter, light and gas distributions, that will lead us to the distribution of substructures within the MACSJ0416, and Abell 2744 vicinities. Finally I will discuss the different clues that these observables provide on the evolution processes in massive galaxy clusters, and more globally on Lambda-CDM. If time permits, I will discuss one of the most beautiful HFF discovery, SN Refsdal, the first multiply-lensed supernovae discovered by Kelly et al. (2015) in MACSJ1149. This particular event was a unique chance to test our mass modeling techniques, and improve our methods. I will thus give an overview of the lensing community’s work on SN Refsdal, and discuss the appearance of its last multiple image.
Friday 25 November 2016
Title : What do numerical simulations tell us about the stellar magnetic field ?
Speaker : Laurène Jouve (IRAP, Toulouse)
Abstract : In this talk, we will review some aspects of the stellar magnetism and in particular what numerical simulations tell us about the physical processes underlying the observations. In cool Sun-like stars, a convective dynamo is thought to be responsible for the presence and evolution of magnetic fields. One important aspect of stellar dynamos is the possible presence of magnetic cycles, whose caracteristics depend on the stellar parameters. Another question is how magnetic flux is transported from the interior to the surface to produce starspots and whether those spots play a role in the dynamo process. In hotter and more massive stars, the manifestations of a magnetic field aredrastically different. Only 5 to 10% of those stars seem to exhibit a strong dipolar magnetic field while most of the other stars are thought to possess a weak complex field. The reason for the existence of this magnetic dichotomy could be linked to the development of magnetic instabilities. We will show in this talk that numerical simulations may help us tackle several of the above key questions about stellar magnetism.
Friday 18 November 2016
Title : Galaxy Evolution in the first billion years with the JWST
Speaker : Andrew Bunker (Oxford University)
Abstract : Over the past ten years, we have identified star-forming galaxies at z>6, within the epoch of reionzation, using the Lyman break technique with deep fields from the Hubble Space Telescope and ground-based imaging. From the inferred rest-frame UV luminosity function, we can address the potential role of star formation in the reionization of the Universe. However, spectroscopic follow-up has been challenging, with Lyman-alpha (the main feature accessible from ground-based observations) often weak or absent. James Webb Space Telescope will revolutionise this - the near-infrared spectrograph NIRSpec works out to 5microns with huge multiplex, sampling the rest-frame optical out to z approximately 6, and potentially obtaining redshifts from [OII] out to z approximately 12 (and much higher with the Lyman break). I will describe the Guaranteed Observing Time programme with my colleagues on the NIRSpec instrument science team. Through emission lines and SED fitting, these observations have the potential to chart the evolution of the star formation rate, dust extinction, metallicity and stellar initial mass function within the first billion years.
Friday 4 November 2016
Title : The Physical Conditions of Galactic Outflows
Speaker : John Chisholm (Geneva University)
Abstract : Strangely, a majority of the baryons in the universe are not in galactic disks ; rather they reside in metal-enriched halos that extend more than 100 kpc from star forming regions. Metal enrichment implies that a portion of the gas in these halos originated in star-forming regions, and has been transported into the halo through galactic outflows. Historically, it has been challenging to observe these galactic outflows because they are diffuse, multiphase structures. Here, I present results from a recent complication of nearby star-forming galaxies with ultraviolet spectra from the Hubble Space Telescope. I use a diverse set of absorption lines to find shallow correlations between the outflow velocity and stellar mass of the host galaxy. Additionally, the equivalent widths of the outflows imply that the observed outflows are photoionized, and I use these photoionization models to estimate the density, metallicity, ionization structure, and size of the outflows. In particular, I find that the outflows are metal-rich and originate much closer to star-forming regions than typically assumed. Finally, I use the line profiles to explore the acceleration of outflow, radial density profile, and the mass outflow rate. I conclude with implications for the enrichment of the circum-galactic medium by these outflows.
Friday 28 October 2016
Title : Galaxy stellar mass assembly from deep imaging surveys
Speaker : Olivier Ilbert (LAM Marseille)
Abstract : A clear and comprehensive picture describing the physical processes which regulate the star formation is still missing in galaxy formation scenario. Stellar mass assembly in galaxies is believed to result from several physical processes such as star formation from in-situ or accreted gas, major or minor mergers, as well as supernovae and AGN feedbacks. The relative contribution and operating timescales of these different processes are still a matter of debate. I will present some observational constrains that we established using large and deep imaging surveys, in particular in the COSMOS field. I will show how these data help us to better constrain the quenching processes and the galaxy growth rate out to z approximately 6.
Friday 14 October 2016
Title : Highlights from GLASS : Probing Galaxy Evolution from Redshift 0 to 8 with Slitless HST Spectroscopy of Lensing Clusters
Speaker : Kasper Borello Schmidt (AIP Potsdam)
Abstract : The Grism Lens-Amplified Survey from Space (GLASS) is a large HST cycle-21 program which targeted 10 massive galaxy clusters all with extensive HST imaging from CLASH and the Hubble Frontier Field Initiative. GLASS has obtained 140 orbits of primary near-infrared WFC3 grism and 140 orbits of parallel optical ACS grism observations. This has resulted in spatially resolved spectroscopy of thousands of galaxies in and behind the 10 galaxy clusters. I will present GLASS and a selection of our most recent results including a census of Ly alpha emission at z>6, spatially resolved star formation maps and metallicity gradients at z approximately 2, and an assessment of the environmental effect on cluster members at z approximately 0.5. These results all rely on high fidelity cluster lens models and have spawned a variety of ground-based follow-up campaigns. I will conclude by highlighting results from our follow-up campaigns which include the detection of the most distant multiply imaged galaxy, the first spectroscopic confirmation of a typical galaxy at z>7.5, and the discovery of [CII] emission from a faint low-metallicity system at z approximately 7.
Friday 7 October 2016
Title : ALMA Observations of the Frontier Fields
Speaker : Franz Bauer (Pontifica University)
Abstract : The Frontier Fields Legacy Program targets six strong lensing clusters with deep HST and Spitzer imaging to detect and characterize the faint background galaxy population, particularly the first galaxies at z between 6-10. Given the wealth of sensitive ancillary data, we initiated an ALMA survey to produce “shallow” roughly 2’x2’ maps of the Frontier Fields at 1.1mm to study intrinsically faint dusty star-forming galaxies (DSFGs) and place constraints on the star formation from a variety of interesting extragalactic source populations (including those at z>6). The first three (A2744, MACS0416, and MACS1149) have now been completed, yielding relatively uniform maps that pinpoint cool dust emission from powerful DSFGs at z>1. I will present a census of the detected objects thus far, as well as some early constraints our team is able to place on the average emission from undetected source populations.
Friday 23 September 2016
Title : Outflows from high redshift quasars : winds and radiation
Speaker : Tiago Costa (Leiden)
Abstract : The detection of bright quasars at redshift 6 - 7 indicates that supermassive black holes of masses in excess of one billion solar masses were already in place when the Universe was less than 1 Gyr old. The rapid growth of such massive systems puts constraints on models of black hole growth and feeding, which must proceed efficiently despite AGN and stellar feedback processes. In this talk, I will present a suite of cosmological hydrodynamic simulations performed with a variety of codes (AREPO/RAMSES-RT/GADGET3) addressing black hole growth and AGN feedback in a range of cosmological environments at z = 6, ranging from average to the most over-dense regions in the Universe. I will argue that the formation of bright quasars at z = 6 is possible provided the black holes grow in very massive haloes, where gas inflow is so efficient that the black holes are able to sustain growth at their Eddington rate. Focus will then be placed on the effects of AGN radiation and winds on the medium surrounding the accreting black holes with an emphasis on the generation of large-scale galactic outflows. Specifically, I will argue that if AGN are to regulate their growth and significantly suppress star formation in their host galaxies, the outflows they launch must be energy-, as opposed to momentum-driven. The theoretical findings will be compared with existing observational detections of molecular outflows at z > 6.
Friday 16 September 2016
Title : VLT/SPHERE deep planet search in the transitional disk of SAO 206462
Speaker : Anne-Lise Maire (MPIA)
Abstract : The SAO 206462 (HD 135344B) disk is one of the few known transitional disks showing asymmetric features in both scattered light and thermal emission. Giant planets in formation have been suggested for accounting for the disk morphology. We obtained new high-contrast and high-resolution near-infrared images of the target with the VLT planet finder instrument SPHERE. The spiral arms and the outer rim of the inner cavity are revealed at high sensitivities without the need for image post-processing. We do not detect any close-in companion candidates. As for the derivation of the detection limits on putative giant planets embedded in the disk, we show that the knowledge of the disk aspect ratio and viscosity is critical for the estimation of the attenuation of a planet signal by the protoplanetary dust because of the gaps that these putative planets can open. Given the assumptions on these parameters, the degradation of the mass limits can vary from negligible to several Jupiter masses at separations beyond the disk spiral arms. Our deep detection limits provide new constraints on a few recent predictions of massive planets in the outer disk based on the spiral density wave theory.
Friday 09 September 2016
Title : Correction of apertures discontinuities for the direct imaging of exoplanets and circumstellar disks
Speaker : Johan Mazoyer (STScI)
Abstract : More than 3000 thousands exoplanets have been discovered to date, but only a few have been imaged directly. However, by allowing the observation of circumstellar disks and planets (sometimes simultaneously around the same star, as in the case of β-pictoris), this method is a fundamental tool for the understanding the process of planetary formation. In addition, direct access to the light of the detected objects allows spectroscopy, paving the way to the full chemical analysis of exoplanets’ atmosphere and disks grains. Several coronagraphic instruments are currently observing to images of young Jupiters and/or Kuiper like disks. These instruments use coronagraphs optimized for circular, often un-obstructed apertures. Indeed, the remaining aberrations created by the atmosphere or optics defaults is limiting the contrast at levels far above the ones created by apertures discontinuities (inter-segment gap or secondary mirror mounts). However, the next generation of ground and space based telescopes will have to address the problem of apertures discontinuities in coronagraphy, if we want to obtain images and spectra of earth sized planets or dust grains below the snow line. In this talk I will present my current research at the Space Telescope Science Institute to improve the contrast level of coronagraphs in the presence of apertures discontinuities using deformable mirrors. I will also quickly present my work in the field of the post processing of high contrast images of circumstellar disks.
Friday 22 July 2016
Title : Nebular diagnostics with MUSE
Speaker : Fernando Selman (ESO)
Abstract : I will present a survey of the literature about different emission-line diagnostic for low-redshift galaxies relevant for the MUSE spectral range, with an application for IC3418, one of the prototypical ram-pressure stripped disk galaxies.
Friday 01 July 2016
Title : Two problems of IPT 2016 : Popsicle Stick Cobra and Sultry Day
Speaker : Jeremy Sautel (ENS)
Abstract : International Physicists Tournament is a world-wide competition which involves undergraduate and master students. A list of 17 open physics problems (http://2016.iptnet.info/list-of-problems/) was proposed to the different teams this year. Jérémy Sautel, from the team "France - ENS Lyon", which won IPT 2016, will present his team’s work on two topics : 1 - Popsicle Stick Cobra : speed and hight of a popsicle stick bomb creted by a weave made of wood and PVC sticks, and 9 - Sultry Day : how to determine the temperature of a hot plate by optics ?.
Friday 24 June 2016
Title : Deriving the gas and dust structure of transition disks from multi-wavelength observations.
Speaker : Andres Carmona (IRAP, Toulouse)
Abstract : Transition disks are protoplanetary disks that display cavities and gaps in the dust distribution. The origin of those features is matter of intense debate in the literature, particularly because there is the exciting possibility that the dust cavities and gaps could be the signpost of planets inside the disk. One key aspect to distinguish between different proposed scenarios for the origin of transition disks is to determine the content and distribution of the gas in the disk, most specifically the gas inside the dust cavity.
In this talk, I will discuss the strengths and limitations of different observational methods that we use to investigate the gas and dust in protoplanetary disks, and I will present how multi-wavelength (IR to sub-mm) multi-technique (imaging, interferometry, high-resolution spectroscopy) observations coupled to radiative transfer modeling can be used to derive the gas and dust disk’s structure. I will illustrate the discussion with the case studies of two transition disks (HD 135344B and HD 139614). Finally, I will conclude with exposing open issues and future perspectives.
*Monday* 20 June 2016
Title : Energy dependence of aperiodic variability in low mass X-ray binaries
Speaker : Holger Stiele (National Tsing Hua University, Taiwan)
Abstract : Black hole and neutron star X-ray binaries show variability on time-scales ranging from milliseconds to years. While variability on the time scale of years is related to the accretion rate of matter, the origin of variability on short time scales is still under debate. In the last two decades a detailed phenomenological picture of short-term variability in low-mass X-ray binaries has emerged, mainly based on RXTE observations that cover energies above 3 keV. This picture comprises periodic or quasi-periodic variability, seen as spikes or humps in power density spectra, that are superposed on broad noise components. The overall shape of the noise components as well as the occurrence of quasi-periodic oscillations is known to vary with the state of the X-ray binary. Quasi-periodic oscillations can be related to the relativistic precession of the accretion flow, instabilities in the accretion disc, or to nuclear burning on the neutron star surface. We are accomplishing a comprehensive s
tudy of archival XMM-Newton observations of black hole and neutron star low-mass X-ray binaries to investigate the variability properties of these sources at softer energies where the thermal disc component starts to emerge.
Here we present the results of a comprehensive variability study of a sample of black hole X-ray binaries, including a discussion of the energy dependence of the variability. We will discuss the implications of these findings for the picture of the accretion geometry in black hole X-ray binaries.
Regarding neutron star binaries, we performed a variability study of archival XMM-Newton data of 4U 1636-536 and investigated the energy dependence of the low frequency variability. Here we present the results of our waveform analysis and phase resolved spectral investigations of the low frequency variability in this source and discuss implications on the thermonuclear burning mode, neutron star size and equation of state.
*Monday* 13 June 2016
Title : Balancing the Baryon Budget
Speaker : Ann Zabludoff (Arizona)
Abstract : The deep gravitational potential wells of clusters of galaxies should capture fair samples of the total baryon fraction of the Universe, unless other physical processes drive baryons out of clusters. Thus precision measurements of the baryon fraction, particularly as a function of cluster mass, can reveal the history of baryon flux into and out of clusters. How those baryons are then apportioned between stars and intracluster gas---the star formation efficiency---informs models of cluster assembly and massive galaxy evolution, as well as efforts to use the cluster gas fraction to constrain the mass density and dark energy equation of state parameters. Even the partitioning of the stellar baryons alone, in and out of galaxies, tests models of cluster galaxy evolution, as intracluster stars are the final, unambiguous signature of stars stripped from cluster galaxies during tidal encounters. We have discovered that intracluster stars are a significant part of the stellar baryons in clusters and poorer groups of galaxies. I will present new work characterizing the properties of this previously unexplored component, as well as the consequences for the cluster baryon budget and its relationship to the Universal value.
Friday 10 June 2016
Title : Growing black holes in growing galaxies
Speaker : Marta Volonteri (IAP)
Abstract : Massive black holes, weighing millions to billions of solar masses, inhabit the centers of today’s galaxies. Black hole masses typically scale with properties of their hosts, such as bulge mass and velocity dispersion. The progenitors of these black holes powered luminous quasars within the first billion years of the Universe. The first massive black holes must therefore have formed around the time the first stars and galaxies appeared, and then evolved along with their hosts for the past thirteen billion years. I will discuss some aspects of the cosmic evolution of massive black holes, from their formation to their growth and how different physical processes shape the relation between black holes and galaxies.
Friday 03 June 2016
Title : The impact of energetic phenomena on the evolution of galaxies and
their black holes — a theoretical perspective
Speaker : Michaela Hirschmann (IAP)
Abstract : We present new insights from advanced semi-analytic models (SAMs) and modern cosmological simulations indicating a much more complex interplay between different feedback processes and galaxy/black hole (BH) growth than traditionally anticipated. Both, SAMs and large-scale simulations are successful in producing realistic global BH populations e.g. capturing the observed anti-hierarchical trend in BH growth. For that, stellar feedback can play e.g. an important role for the gas accretion onto low-mass BHs at low redshifts. Employing advanced high-resolution zoom simulations, we can confirm and extend previous results that different AGN feedback mechanisms can strongly affect the kinematics and stellar content of massive galaxies, e.g. by regulating their insitu star formation. But contrary to the traditional picture, galactic winds from SN explosions and massive stars are also crucial for the stellar mass assembly of massive galaxies, not only significantly delaying early insitu star formation but also strongly affecting the accreted stellar populations at large radii. Successes and limitations of most recent models and perspectives for future improvements will be given.
Friday 27 May 2016
Title : The SCUBA-2 Cosmology Legacy Survey
Speaker : Dan Smith (University of Hertfordshire)
Abstract : I will present my results from using the SCUBA-2 Cosmology Legacy Survey (CLS) 850 um observations over the UKIDSS Ultra-Deep Survey field to study the evolution of the obscured component of the cosmic star formation rate density over 1 < z < 4. The 850 um data benefit from negative k-correction, and as a result, allow us to probe for star formation in a virtually redshift-independent manner, provided that some assumptions about the choice of far-infrared SED are made. Using a mass-complete sample of K < 24 galaxies, classified as star-forming or passive using their UVJ rest-frame colours, we are able not only to study evolution in the so-called star formation rate "main sequence", but also (by convolving the main sequence with the stellar mass function) the contribution of different galaxy populations to the cosmic star formation rate density.
In addition, I will say a few words about the WEAVE-LOFAR survey, which — starting in 2018 — will obtain more than a million spectra of low-frequency selected radio sources, with a wide range of science goals.
Friday 20 May 2016
Title : Observations of inhomogeneity and backreaction : A status report
Speaker : David Wiltshire, University of Canterbury (New Zealand)
Abstract : The possibility of backreaction - that inhomogeneous structures on small (< 100/h Mpc) scales change average cosmic evolution relative to a standard homogeneous isotropic FLRW model - can be tested in different ways.
1. In our own < 100/h Mpc environment the assumption of a FLRW geometry can be tested model-independently. We present very strong Bayesian evidence that the standard CMB rest frame is not the frame in which the spherically averaged variation of the Hubble expansion is minimized. Using large ray tracing simulations in exact solutions of Einstein’s equations we further show that observations are consistent with a differential expansion of space on < 70/h Mpc scales that is not included in the standard cosmology. This may have testable implications for CMB anomalies.
2. On > 100/h Mpc scales detailed models of backreaction are required for cosmological tests. The Timescape Cosmology provides such a phenomenology. We report on the status of observational tests of this model, including the fitting of the acoustic peaks in the CMB anisotropy spectrum. The Timescape Cosmology passes current tests, and can be distinguished from the standard cosmology with future Euclid satellite data. In terms of the CMB we find that the effects of backreaction in the primordial plasma still need to be accounted for to deal with systematic uncertainties of 8-13% in particular cosmological parameters.
Friday 29 April 2016
Title : Dust coagulation with porosity evolution in planet formation
Speaker : Akimasa Kataoka
Abstract : Dust coagulation is the first process of planet formation. Dust grains stick to other grains to form porous dust aggregates. This porosity evolution is a key to understand the planet formation. We have theoretically revealed the overall porosity evolution from micron-sized dust grains to km-sized planetesimals based ; dust grains form extremely porous dust aggregates where the filling factor is approximately 10^-4, and then they are compressed by their collisions, the disk gas, and their self-gravity. The mass and porosity of the final product is consistent with the comets, which are believed to be the remnants of planetesimals. We have also calculated the opacity of the fluffy dust aggregates and discuss possible way to distinguish between compact and fluffy dust aggregates.
Friday 22 April 2016
Title : Atmospheric flows in Stellar to Substellar Atmospheres and their impact on dust formation
Speaker : France Allard (CRAL)
Abstract : Understanding the atmospheric and evolutive properties of brown dwarfs and gas giant exoplanets have been important challenges for modelers around the world since the discovery of the first brown dwarfs in the field (Nakajima et al. 1995) and in the Pleiades cluster (Rebolo et al. 1995) and exoplanet (Mayor & Quéloz 1995). The early studies of brown dwarfs have provided insights into atmospheric physics, with discoveries ranging from cloud formation (Tsuji et al. 1996), methane band (Oppenheimer et al. 1995) and ammonia band (Delorme et al. 2008) formation, to the formation of quasi-molecular KI-H2 absorption (Allard et al. 2007), and to disequilibrium chemistry (Yelle & Griffith 2001). Cloud formation is a primary challenge to the understanding of the spectral transition between the stellar and planetary mass regimes, and play a crutial role for the spectral distribution of very low mass stars and brown dwarfs cooler then 3000K, for biosignature detectability, and for planet habitability. Recently, Crossfield et al. (2014) have demonstrated by Doppler imaging the presence of important surface inhomogeneities at the surface of a close-by brown dwarf (2.0 parsecs) brown dwarf in the absorption bands of carbon monoxyde at 2.2 micrometers. And we have been able to determine by radiation hydrodynamical simulations the formation of laminar flows in these atmospheres with periods of a few hours, that could explain the observed spectral and photometric variability of these objects (Freytag, Allard & Homeier 2016, en prép.). I will review the progress achieved in reproducing the spectral properties of very low mass stars, brown dwarfs/gas giant exoplanets, and review progress in modeling more accurately their atmospheres using Radiation HydroDynamical (RHD) simulations.
Friday 8 April 2016
Title : Shear-driven instabilities and shocks in the atmospheres of hot Jupiters
Speaker : Sebastien Fromang (CEA)
Abstract : Hot Jupiters are giants gaseous planets orbiting very close to their central star. Because of the extreme irradiation they receive, the dynamics of the atmosphere is different from that of the solar system giant planets. In the past few years, general circulation models of the atmosphere of hot Jupiter have shown in particular the existence of a supersonic eastward equatorial jet. Yet these results have mostly been obtained using numerical schemes that filter out vertically propagating sound waves and assume vertical hydrostatic equilibrium,raising the issue of the jet stability and the possible presence of shocks.
In this talk, I will present the results of a series of high resolution simulations specifically designed to address these issues. I will show that the jet is indeed destabilized by two instabilities of Kelvin-Helmoltz type respectively associated with the meridional and vertical velocity shears. These instabilities induce significant flow variability and weak shocks in the upper layers of the atmosphere and may affect their temperature-pressure profile as well as the internal structure of hot Jupiters.
Friday 25 March 2016
Title : Spectroscopic Studies of Star-Forming Galaxies in the Reionization Era
Speaker : Richard Ellis (ESO and UCL)
Abstract : Deep exposures with the Hubble Space Telescope (HST) have provided the primary evidence that star-forming galaxies were present in the first billion years of cosmic history. Sometime during this early period the intergalactic medium transitioned from a neutral gas to one that is fully ionized. How did this `cosmic reionization’ occur and were star-forming galaxies responsible ? Imaging of deep fields with HST’s Wide Field Camera 3 in conjunction with Spitzer photometry and Keck spectroscopy has provided important new insight into understanding when reionization occurred and the role of early galaxies in the process. Recent Planck results on the optical depth of electron scattering to CMB photons provide complementary information. I will review this rapid progress in our understanding of the last missing piece in our overall picture of cosmic history and discuss the remaining challenges ahead of future facilities such as JWST and E-ELT.
Friday 18 March 2016
Title : In-situ energetic particle acceleration in young stellar objects : Confronting theory to observations
Speaker : Alexandre Marcowith (Montpellier)
Abstract : Recent observations in some well-observed low to intermediate solar mass young stellar objects conclude to ionization rates that can be several orders of magnitude higher that local interstellar values. Also some non-thermal, likely synchrotron, radiation have been detected from hot spots in association with the object DG-Tau. In these high density column media, such ionization rates appear difficult to explain by only invoking UV or X-ray radiation. Simple energetic arguments show that background interstellar cosmic rays can not penetrate so deep in these environments because they suffer too high ionization losses. We have conducted calculations of in-situ energetic particle acceleration adapting the diffusive shock acceleration theory developed in the context of fast moving supernova remnant shocks to the case of partially ionized environments found in young stellar environments. We show that particle acceleration is possible in some sites, namely, along the jet and at the termination shock and at the shock stellar surface. We have obtained particle distribution spectra, accounted for all types of losses, calculated propagated spectra in the hot spot or in the envelope and tested our calculations with respect to the observations. We found a relatively good agreement between theory and observations. Acceleration appears to be an intermittent process, in some conditions the maximum particle energies can be as high as several hundred of GeV or even a few TeV. One may expect that some of these objects to be targets of the future Cherenkov telescope CTA. Some aspect of the model should also be testable using ALMA high angular resolution observations.
Friday 11 March 2016
Title : The Star Formation Activity and Its Diversity of Low-Mass Galaxies at Cosmic Noon
Speaker : Haruka Kusakabe (University of Tokyo)
Abstract : Low-mass galaxies at high redshift play a key role in galaxy formation and evolution as building blocks of more massive galaxies seen at later epochs. I will talk about star formation properties and their diversity of Lya Emitters (LAEs) at z = 2.2, a commonly seen low-mass galaxy population. First, by stacking deep Spitzer/MIPS and Herschel/PACS images for 213 LAEs in the GOODS-South, we find for the first time that LAEs typically have very low IR luminosities less than L3σ = 1.1 χ 10^10 Lsun (3 σ upper limit) and that TIR their attenuation curve is consistent not with the Calzetti curve but with the SMC curve (Kusakabe et al. 2015, ApJ, 800, L29). Second, we divide 604 LAEs in the SXDS field into sub-samples based on the distribution of four UV physical parameters : MUV, β , LLya, and EWLya,r. Then, we calculate the halo mass and stellar population parameters for each sub-sample from clustering analysis and SED fitting with SMC curve, respectively. While two thirds of the entire sample are on the SFMS with stellar masses of roughly 10^9 Msun, the remaining one third, which are the lowest-mass objects in our sample, are forming stars burstly with stellar masses of roughly 10^7 Msun with SFRs exceeding the baryon accretion rates of their hosting halos. These low-stellar mass LAEs may be in initial forming phases at cosmic noon (Kusakabe et al. in prep).
Friday 4 March 2016
Title : Can we describe galaxy formation with a merger tree and a set of coupled, ordinary differential equations ?
Speaker : Peter Mitchell (CRAL)
Abstract : Semi-analytic galaxy formation models are an established tool for describing how galaxies assemble their stellar mass within the context of hierarchical structure formation. With the advent of well calibrated hydrodynamical simulations of cosmologically representative volumes, it is timely to critically review the assumptions and resulting predictions of semi-analytic models. In this seminar, I will focus primarily on the relationship between stellar mass and halo mass assembly, and the importance of accurately modelling gas cycling as a consequence of supernovae explosions. I will also discuss the implications of apparent tensions between observational estimates of stellar masses and star formation rates.
Friday 26 February 2016
Title : Image Restoration Methods : From Numerical Optimization Strategies to Blind Deconvolution and Shift-variant Deblurring, which will include three subtopics
Speaker : Rahul Mourya (Université Jean Monnet, Saint-Etienne)
Abstract :
i) Introduction describing image restoration problem as optimization problem
ii) An approach for astronomical image restoration via image decomposition
iii) Shift-variant Image Deblurring.
Friday 12 February 2016
Title : Dark energy as emerging average negative curvature : low-redshift observational challenges
Speaker : Boud Roukema (Torun, Poland)
Abstract : The hypothesis that recently emerging average negative curvature provides the main physical parameter that replaces dark energy is at least seven years old. This hypothesis offers a physically conservative alternative to the cosmological constant and "new physics" hypotheses, since the physical assumption is to apply general relativity during the structure formation epoch rather than assume rigid co-moving space (one of the assumptions underlying LambdaCDM). A brief pedagogical overview of the standard model and of scalar averaging will be presented. Low-redshift observational parameter values implied by a bi-domain implementation of scalar averaging will be derived. Observational prospects for measuring these parameters will be discussed. Conceptual analogies will be presented as tools for shifting from Euclidean co-moving space intuition to curved space intuition.
Friday 29 January 2016
Title : An overview of the mid-infrared spectro-interferometer MATISSE : science, concept, and current status
Speaker : Alexis Matter (Nice)
Abstract : MATISSE is the second generation mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This new interferometric instrument will allow significant advances by opening new avenues in various fundamental research fields : studying the planet-forming region of disks around young stellar objects, understanding the surface structures and mass loss phenomena affecting evolved stars, and probing the environments of black holes in active galactic nuclei. As a first breakthrough, MATISSE will enlarge the spectral domain of current optical interferometers by offering the L & M bands in addition to the N band. This will open a wide wavelength domain, ranging from 2.8 to 13 microns, exploring angular scales as small as 3 mas (L/M band) / 10 mas (N band). As a second breakthrough, MATISSE will allow mid-infrared imaging - closure-phase aperture-synthesis imaging - with up to four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. Moreover, MATISSE will offer a spectral resolution range between R approximately 30 to approximately 5000. Here, i introduce the physical concept of MATISSE including a description of the signal on the detectors and an evaluation of the expected performances. I also present the potential of MATISSE for the study of the inner regions of protoplanetary disks, which is one of the main science cases that has driven the instrument design and motivated several VLTI upgrades (GRA4MAT & NAOMI). Finally, i discuss the current status of the instrument, which is entering its testing phase, and the foreseen schedule for the next two years that will lead to the first scientific exploitation in 2018.
Friday 22 January 2016
Title : The assumptions we make when we talk about attenuation
Speaker : Kyle Penner (CEA)
Abstract : We use rest-frame infrared, ultraviolet, and Ha line luminosities of dust-poor and dusty galaxies at z approximately 0 and z approximately 1 to compare measured Ha attenuation values with those predicted by the Calzetti attenuation formula. The predictions, based on UV attenuation values, overestimate the measured Ha attenuation values for all samples. The interpretation of this result for dust-poor galaxies at z approximately 0 is that regions with ionizing stars have more dust than regions with nonionizing UV-emitting stars. Dust distributions for these galaxies are nonuniform. The interpretation of the overestimates for dusty galaxies at both redshifts is less clear. If the attenuation formula is inapplicable to these galaxies, perhaps the disagreements are unphysical ; perhaps dust distributions in these galaxies are uniform. If the attenuation formula does apply, then dusty galaxies have nonuniform dust distributions ; the distributions are more uniform than they are in dust-poor galaxies. A broad range of Ha attenuation values at a given UV attenuation for dusty galaxies at z approximately 1, if real, indicates diverse dust morphologies and the implausibility of the screen assumption.
Friday 15 January 2016
Title : Low Frequency view of merging galaxy clusters
Speaker : Mamta Pommier (CRAL)
Abstract : Cluster of galaxies are the largest gravitationally bounded systems in our Universe. They emit non-thermal emission during merger events that takes the form of cluster wide (Mpc-size) radio halos, mini-halos and relics, further implying the presence of relativistic particles and magnetic fields within the intracluster medium. These sources reveal a population of ultra-relativistic electrons coexisting with the intracluster thermal gas. The origin of these relativistic electrons is one of the most intriguing problems of cluster astrophysics. Further, the radio observations also show presence of μG level magnetic fields within the galaxy clusters, with values up to tens of μG at the center of cooling core clusters. In this presentation, I will discuss about the new results on the low frequency nature of a few galaxy clusters observed with the GMRT, VLA and LOFAR as well as their statistical properties. I will also outline the important contribution that are expected in this area from sophisticated radio facilities (like LOFAR, SKA, etc.) as well as synergies with ongoing survey with Multi Unit Spectroscopic Explorer (MUSE).