CRAL@Obs seminar by Pierre Ocvirk (Observatoire de Strasbourg)
2025-06-13 11:00 - 12:00 (Europe/Paris) | Observatoire
Towards simulating the Epoch of Reionization on exascale supercomputers —
The Epoch of Reionization (EoR) represents a fundamental phase transition in cosmic history, demanding sophisticated numerical simulations to interpret observations of the high-z Lyman alpha forest, combined with HST/JWST, and upcoming 21cm surveys. Our latest Cosmic Dawn III simulation has redefined the state-of-the-art by simultaneously and precisely predicting multiple EoR observables: the evolution of the neutral Hydrogen fraction, Thomson scattering optical depth, ionizing photon rates, Lyman-α forest optical depth distributions, and ionizing mean free path evolution. However, critical limitations persist, including insufficient spatial resolution for internal galaxy physics and box sizes still to small for interpreting observed volumes like ASPIRE/EIGER. Breaking through these barriers requires exascale computing and new computational frameworks, which I will illustrâtes. Additionally, I will discuss the underexplored limitations of the M1 radiative transfer model, presenting new work by PhD student Mei Palanque on Pn methods coupled to photochemistry, which quantifies M1 artifacts and opens pathways to more accurate EoR modeling.

CRAL@Obs seminar by Rahna Payyasseri Thanduparackal (CEFCA, Teruel)
2025-07-04 11:00 - 12:00 | Observatoire
Mapping Extended Circumgalactic Lyα Nebulae Around Quasars With J-PAS —
Circum-Galactic Medium (CGM) is the gas surrounding galaxies outside their disksand inside their virial radii. Lyman-alpha (Lyα) emission is one of the most important spectral features for probing the CGM at high redshifts, providing crucial insights into the formation and evolution of galaxies and their environments in the early Universe. This emission can trace outflows, inflows, and other processes that are key to understanding how galaxies interact with their surroundings. However, the origin of Lyα emission around quasars still under debate, particularly in terms of whether the emission arises primarily from photoionized gas, from cooling gas in the CGM, or from processes related to quasar-driven outflows. These different scenarios have profound implications for our understanding of galaxy formation and evolution, as they can shed light on feedback mechanisms, gas accretion, and the impact of quasars on their host galaxies. In this talk, I will present a study of extended Lyα emission around quasars at z>2 utilizing the capabilities of the Javalambre Physics of the Accelerating Universe Astrophysical Survey (J-PAS) with its 54 optical narrow-band filters over thousands of degrees. I will highlight our recent discovery of a massive Lyα nebula using J-PAS early data release, demonstrating the unique ability of J-PAS’s multi-band photometry to detect faint, extended structures, providing new perspectives on the dynamic relationship between quasars and their environments at high redshift.

CRAL@ENS Seminar Valentin Le Gouellec
2025-07-08 14:00 - 15:30 | ENS
Title : Evolution of embedded protostars studied with submillimeter and near-infrared observations
Abstract :
Sun-like stars are thought to accrete most of their final mass during the protostellar phase, where the protostellar embryo is surrounded by an infalling dense envelope. The so-called Class 0 phase designates the youngest protostellar stage, where the accretion is thought to be the most vigorous. Several of the final outcome of star-disk systems (multiplicity, stellar parameters, disk properties) are thought to originate from this protostellar phase. I will present several aspects of the processes that are thought to be at play during this phase, using both observations (ALMA, JWST, Keck) and modeling tools (MHD simulations, radiative transfer). In the (sub-) millimeter, ALMA reveals the detailed structure of magnetic field lines in the inner envelope of collapsing protostars via the observation of the linear polarization of thermal dust emission.
We examine the potential role of magnetic fields in controlling the mass infall rate and governing the evolution of the nascent circumstellar disk.
One question we seek to answer is whether more disorganized magnetic fields and infalling density structures correspond to higher probability of variable accretion and/or fragmentation.
Modeling of these structures via simulations and radiative transfer calculations of dust polarized emission also allows us to explore the properties of aligned dust grains in young protostellar envelopes, where we tend to probe early dust grain evolution.
We will complete this view of protostellar accretion/ejection with recent near-IR observations of Class 0 protostars. In rare cases the blueshifted cavity created by the outflow is sufficiently close to pole-on to liberate enough near-infrared scattered light for us to probe the immediate surroundings of the central object. Bry, several H$_2$ and CO overtone lines are detected and analyzed. Detecting Bry in several sources suggests a good fraction but not all of Class 0s accrete via magnetospheric accretion. A significant higher fraction of Class 0 objects tend to exhibit the CO overtone bands in emission than in archival K-band spectra of later-evolved, Class I protostars, confirming that the mass accretion rate is on average higher in the most embedded phase. Recent JWST observations and their synergy with ALMA are constraining even further the accretion/ejection mechanisms and disk formation processes of young protostars. They allow a complete study of molecular and ionized gas excitation and kinematics in the vicinity of protostars. Analyzing these properties toward the most embedded protostars, whose IR spectra is now unveiled thanks to the sensitivity of JWST, shall sharpen our understanding of the evolution of accretion and ejection mechanisms with time.

CRAL@ENS Seminar - David Whitworth
2025-09-12 11:00 - 12:00 | ENS
David Whitworth is a new postdoc at the ENS.

CRAL@ENS Seminar - Clement Bonnerot (University of Birmingham)
2025-09-23 11:00 - 12:00 | ENS
Unleashing the predictive power of tidal disruption events
The tidal force from a supermassive black hole can disrupt a star passing too close, resulting in a powerful electromagnetic flare as the stellar debris fuels the compact object. Such tidal disruption events provide a unique window into the otherwise hidden population of quiescent supermassive black holes and can offer clues to the mystery of their formation mechanism in the early Universe. With the advent of the Rubin Observatory, we are entering an observational golden era with thousands of events expected to be discovered, increasing our current sample by two orders of magnitude. Fully exploiting this potential requires a robust theoretical framework that characterizes the observational signatures produced and links them to the properties of the black hole and disrupted star. I will present progress toward this goal, which relies on a suite of interlinked simulations to follow the entire evolution of tidal disruption events and predict their electromagnetic emission.

HDR defense Jérémy Fensch @ ENS
2025-09-25 14:00 - 16:30 | ENS
En Salle des thèse à l’ENS.

CRAL@ENS Seminar - Stefanie Walch-Gassner (University of Cologne)
2025-09-26 11:00 - 12:00 | ENS
The impact of stellar feedback on galaxies
Small-scale stellar feedback can impact the large-scale evolution of galaxies by regulating star formation and by driving galactic fountain flows and outflows. Using modern high-performance computing simulations, we can study the relative importance of stellar winds, radiation, and supernovae in shaping the multi-phase interstellar medium. Using these simulations, we find that pre-supernova feedback is highly relevant for regulating star formation. In particular, the ionizing radiation of massive stars is dominating over the impact of non-ionizing radiation or stellar winds. On the other hand, supernovae drive hot bubbles and super-bubbles with substantially higher pressure than the typical midplane pressure of a disk galaxy, thereby pushing gas out into the circum-galactic medium. Additionally, cosmic rays, which are in this context most importantly accelerated by supernova shocks, help to sustain the galactic outflow via a vertical cosmic ray pressure gradient.

CRAL@Obs Seminar - Jenny Wagner (Helsinki Institute of Physics)
2025-10-08 11:00 - 12:00 | Observatoire
Title: From cosmic lasagna to space-time spaghetti — obtaining the least-model-dependent strong gravitational lensing probe

Abstract: Strong gravitational lensing has advanced as a standard probe to map mass densities of cosmic structures or to try and infer parameters of the cosmological concordance model, like the Hubble Constant.
In this talk, I will introduce those properties of a light-deflecting cosmic structure that can be uniquely and directly determined from observables without any lens model assumptions. These properties constitute the maximum information common to all model-based mass maps and require less than a second of computing time [1]. The derivation of these characteristics also reveals the most general class of lensing degeneracies and a simple physical interpretation of their origins [2].
I will showcase the power of this approach in two galaxy clusters: we can infer a least-model-dependent smoothness scale for dark matter, reconstruct the morphology of the lensed source galaxy, and even infer global properties of the lensing cluster in combination with complementary data [3,4]. Thus, with an increasing amount of data, this approach will allow us to gain a deeper understanding of the _necessary_ amount and properties of dark matter in cosmic structures.
References:
[1] https://arxiv.org/abs/1906.05285
[2] https://arxiv.org/abs/1809.03505, https://arxiv.org/abs/1904.07239, https://arxiv.org/abs/2203.06190
[3] https://arxiv.org/abs/2105.04562, https://arxiv.org/abs/2207.01630
[4] https://arxiv.org/abs/2306.11779

CRAL@ENS Seminar - Henrik Beuther (MPIA Heidelberg)
2025-10-10 11:00 - 12:00 | ENS

CRAL@Obs Seminar - Sophia Stacevic (LIRA, Paris)
2025-10-17 11:00 - 12:00 | Observatoire

CRAL@Obs Seminar – Seok-Jun Chang (MPIA)
2025-10-24 11:00 - 12:00 | Observatoire
Title: Probing Cold Gas around Galaxies through the Mg II Resonance Doublet
Abstract: The structure and kinematics of the gas around and in galaxies are crucial for understanding the multiphase flows within the galactic ecosystem and, thus, galaxy evolution and star formation. Recent advancements in instruments and techniques offer a new perspective on circumgalactic gas flows through emission lines such as Hydrogen Lyman-α and resonance lines of metals (e.g., Mg II, C IV, O VI, and N V). In particular, the Mg II λλ2796, 2803 doublet has emerged as a promising tracer of cold gas at T ~ 10^4 K. To decode the message carried by the Mg II doublet emission, we developed a 3D Monte-Carlo radiative transfer code ‘RT-scat’. In this talk, I will present simulated results from ‘RT-scat’, demonstrating how Mg II lines form in various environments, including inflowing/outflowing, smooth/clumpy, and dusty gas. In particular, I will also highlight how the MgII doublet line ratio can serve as a potential indicator of LyC escape. Furthermore, I will introduce our new radiative transfer modeling of Mg II emission halos around star-forming galaxies at z ~ 1 from the MAGG and MUDF survey. Our results indicate the presence of slowly moving or outflowing cold gas and reveal strong anisotropies in its distribution. If time allows, I will also discuss other metal resonance lines as tracers of multiphase gas.

CRAL@ENS Seminar - Adnan Ali Ahmad (AstroENS)
2025-11-21 11:00 - 12:00 | ENS

CRAL@Obs Seminar - Mathias Nowak (LIRA, Paris)
2025-12-05 11:00 - 12:00 | Observatoire
Title:
Results and perspectives from the ExoGRAVITY Large Programme

Abstract:
The direct detection of a giant exoplanet using long-baseline interferometry in 2019 marked the debut of the GRAVITY instrument in exoplanet research. This breakthrough was followed by the detailed spectroscopic characterization of beta Pictoris b’s atmosphere in 2020, demonstrating the potential of long-baseline interferometry for studying young giant planets. These successes led to the establishment of the ExoGRAVITY Large Programme on the Very Large Telescope Interferometer (VLTI), dedicated to the characterization of young giant planets. Fast forward five years, and this innovative method has yielded a wealth of discoveries, including the direct confirmation of beta Pictoris c, the detection of brown dwarf companions inferred from Gaia data, and the resolution of the « underluminous » brown dwarfs puzzle. The implementation of GRA4MAT has expanded observation capabilities to include longer wavelengths (L to N bands), while the GRAVITY+ upgrade promises enhanced contrast at small separations in K-band. Initiatives are now underway to explore shorter wavelengths (J and Y bands) at the VLTI, offering strong potential for exoplanet studies. The detection of giant planets in reflected light, rather than thermal emission, is quickly becoming the next frontier in high-contrast interferometry, inspiring more ambitious ideas, such as adding a new Unit Telescope (UT) to the VLTI array. In this talk, I will review the key results of the ExoGRAVITY Large Programme and provide perspectives on upcoming developments in high-contrast interferometry for exoplanet characterization.

CRAL@Obs Seminar - Faustine Cantalloube & Julien Milli (IPAG, Grenoble)
2025-12-12 11:00 - 13:00 | Observatoire
Titre: L’impact du dérèglement climatique sur l’astronomie observationnelle

Résumé: Les grands observatoires astronomiques fonctionnant dans le visible/infrarouge sont souvent accompagnés de station météo afin de suivre les paramètres clés qui affectent la qualité des observations obtenues. La plupart de ces suivis ont maintenant atteint une échelle temporelle de 30ans, permettant d’explorer si il existe des variations climatiques dans les données. L’étude à donc été menée sur de nombreux sites, en particulier à l’observatoire du Cerro Paranal (Chili) qui abrite le Very Large Telescope et l’observatoire du Mauna Kea (Hawaii) qui abrite notamment le Canada France Hawai Telescope. Cependant, un observatoire astronomique est plus qu’un ensemble de quantités physiques, il faut aussi prendre en compte tout un ecosystème qui permet à un observatoire d’opérer de façon fonctionnelle…

CRAL@ENS Seminar Ruediger Pakmor
2026-02-27 11:00 - 12:00 | ENS
TBD

CRAL@Obs Seminar Guillaume Tochon
2026-03-06 11:00 - 12:00 | Observatoire

CRAL@ENS Seminar by Olivier Ilbert (LAM, Aix-Marseille University)-CANCELLED!
2026-03-13 11:00 - 12:00
Title: Stellar mass assembly of massive galaxies in the first five billion years of the Universe

Abstract:Over the past two decades, extensive multi-wavelength observations of the COSMOS field have provided an exceptional view of galaxy evolution across cosmic time. With the recent addition of JWST/NIRCam imaging, our understanding of the distant Universe has advanced dramatically. We can now investigate the physical properties and morphologies of massive galaxies at cosmic noon with unprecedented detail and extend these studies back to the primordial Universe.
After introducing the broader context of galaxy formation and evolution, I will present the confirmation for a population of unexpectedly bright galaxies already in place at z>10, i.e. less than 500 million years of cosmic time. These sources challenge standard models of early galaxy formation. While several explanations have been already suggested, our clustering measurements support a scenario in which star-formation efficiency was significantly elevated in the young Universe.
In the second part of the talk, I will present new results connecting star‑formation histories, morphology, and environment for galaxies at cosmic noon 1 < z < 3. These findings establish solid links between these key aspects of galaxy evolution, and shed new light on the emergence of the massive and quenched elliptical galaxies.

CRAL Seminar @Observatory by Philippe Priolet
2026-04-24 11:00 - 12:00 | Observatoire
Title: The puzzling nature of hot exozodiacal dust: the case of Beta Pictoris
Abstract: Hot exozodiacal clouds are populations of dust grains residing in the inner regions of main-sequence stars at temperatures exceeding 1000K. It is a puzzling phenomena in exoplanetary science. On theoretical grounds, such dust should rapidly sublimate or be expelled by radiation pressure, yet interferometric observations continue to reveal its presence around a growing number of stars. The lack of a satisfying physical model, combined with the intrinsic difficulty of resolving emission just tens of milliarcseconds from the host star, makes every new high-quality dataset particularly valuable.
In this talk, I will begin with a general introduction to exozodiacal dust: its origin, physical properties, and the interferometric techniques used to detect it. I will then present new VLTI/MATISSE observations of Beta Pictoris, one of the most iconic debris disk systems and a prime target for studying the inner regions of a young planetary system. A central challenge in the analysis of interferometric data is the proper handling of correlated noise, and I will describe our approach to estimating correlation matrices for MATISSE data, which is critical for obtaining robust constraints on the dust properties. I will close with an outlook on the future of this field, discussing the prospects offered by nulling interferometry with VLTI/NOTT, and the broader implications of exozodiacal dust for the direct imaging of Earth-like planets with next-generation facilities such as the Habitable Worlds Observatory and the Extremely Large Telescope.

CRAL Seminar @ENS - Francisco Rodrigues Montero (Chicago University)
2026-05-26 11:00 - 12:00 | ENS
Title: CALIMA: Opening the window to the imprint of formation and evolution of H2, CO, dust and PAHs on the ISM
Abstract: The thermodynamical state of the interstellar medium (ISM) in the local Universe is now understood to arise from a tightly coupled network of processes. Much of this progress, however, has been driven by theoretical frameworks that remain incomplete in their ability to self-consistently predict the co-evolution of all these components. Modern galaxy formation simulations have reached a remarkable level of sophistication, yet they typically rely on simplified treatments of radiative processes and of the microphysical mechanisms governing ISM heating and cooling. Conversely, photo-chemical models such as CLOUDY provide a detailed description of interactions between radiation, ions, molecules, and dust, but lack the ability to capture the dynamical, multi-scale nature of the ISM. Understanding the processes regulating the formation and evolution of molecules, dust, and polycyclic aromatic hydrocarbons (PAHs) is therefore crucial for galaxy evolution across cosmic time. These mechanisms remain poorly constrained under the low-metallicity, high star formation rate conditions characteristic of the high-redshift Universe. To address this gap, I will present the first fully self-consistent model of dust, CO, and H₂ implemented within the RAMSES code. This framework is coupled to radiative transfer, non-equilibrium thermochemistry, and a realistic multi-phase ISM description, and explicitly tracks carbonaceous and silicate grains together with PAHs. I will highlight applications ranging from high-resolution simulations of isolated galaxies probing the link between dust and PAH properties and local ISM conditions, to cosmological simulations exploring the origin and evolution of dust in the first galaxies.
Where? ENS Monod, Amphi C

CRAL Seminar @Obs – Vincent Chambouleyron (LAM)
2026-05-28 11:00 - 12:00 | Observatoire
Title: PAPYRUS: transforming a legacy telescope at Observatoire de Haute-Provence into an ELT innovation platform

Abstract:

Modern observatories rely on wavefront sensing and control to achieve their ultimate angular resolution. Adaptive optics (AO) has become a cornerstone of large telescope instrumentation, and all ELT instruments will operate behind an AO system. While AO technologies are now highly mature, several challenges still require further development and on-sky validation, particularly for high-contrast imaging and the direct detection of exoplanets — the main focus of this talk. Progress in this field depends on extensive on-sky experimentation, yet access to large telescopes for instrumentation R&D remains limited. PAPYRUS was created to address this gap by transforming a small legacy telescope at Observatoire de Haute-Provence into a dedicated adaptive optics platform for on-sky technology maturation. Operating from June 2022 to April 2026, PAPYRUS has enabled the testing of innovative wavefront sensing and control concepts under real observing conditions while fostering collaborations across the community. In this presentation, I will review four years of activities on the platform, highlight key technical and scientific results, and discuss the lessons learned from operating a flexible on-sky testbed designed to prepare future AO strategies for the ELT era.

CRAL Seminar @Obs – Olivier Ilbert (LAM)
2026-06-05 11:00 - 12:00 | Observatoire
Stellar mass assembly of massive galaxies in the first five billion years of the Universe

Over the past two decades, extensive multi-wavelength observations of the COSMOS field have provided an exceptional view of galaxy evolution across cosmic time. With the recent addition of JWST/NIRCam imaging, our understanding of the distant Universe has advanced dramatically. We can now investigate the physical properties and morphologies of massive galaxies at cosmic noon with unprecedented detail and extend these studies back to the primordial Universe.

After introducing the broader context of galaxy formation and evolution, I will present the confirmation for a population of unexpectedly bright galaxies already in place at z>10, i.e. less than 500 million years of cosmic time. These sources challenge standard models of early galaxy formation. While several explanations have been already suggested, our clustering measurements support a scenario in which star-formation efficiency was significantly elevated in the young Universe.

In the second part of the talk, I will present new results connecting star‑formation histories, morphology, and environment for galaxies at cosmic noon 1 < z < 3. These findings establish solid links between these key aspects of galaxy evolution, and shed new light on the emergence of the massive and quenched elliptical galaxies.

CRAL Seminar @Obs - Corentin Cudennec
2026-06-12 11:00 - 12:00 | Observatoire
Title: TBD

Abstract:

TBD

CRAL Seminar @ENS – Freeke van de Voort
2026-06-26 11:00 - 12:00 | ENS
Gas flows in the simulated circumgalactic medium —
Feedback-driven outflows are vital for creating realistic galaxies, but feedback models implemented in cosmological simulations vary wildly. The circumgalactic medium (CGM) may be able to break degeneracies and constrain these models. However, there are other differences between simulation efforts, such as which physical processes are included and which numerical method is used. Without understanding their impact on the CGM, we will not be able to confidently constrain galaxy formation models. Using zoom-in cosmological simulations, I will discuss some of the physical and observable properties of the galactic ecosystem. The presence of magnetic fields and feedback from relativistic cosmic rays changes the flow of gas in the CGM, affecting the efficiency of outflows driven by stars or supermassive black holes as well as the turbulence-driven mixing in the CGM. This also affects our simulations’ predictions for neutral hydrogen and metal ions. I will show how these effects of magnetic fields and cosmic rays vary across a wide range in halo mass, from dwarf galaxies to galaxy groups. Finally, I will show preliminary results from our work on thermal conduction in groups and clusters.
