CRAL

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

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

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

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 - 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.

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

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 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@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@Obs seminar by Pierre Ocvirk (Observatoire de Strasbourg)
2025-06-13 11:00 - 12:00 | 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 internal seminar by Rémi Giroud
2025-06-06 11:00 - 12:00
TBD

CRAL@Obs internal seminar by Louis Quilley
2025-05-23 11:00 - 12:00 | Observatoire
Galaxy morphology: how the growth of their bulge shapes the evolution of galaxies —
Almost a century after the publication of the Hubble sequence (Hubble 1926), that classifies galaxies according to their shapes and features, galaxy morphology remains a topical issue. Indeed, the structure of galaxies provide insight on how they acquire their mass, grow their stellar populations, and eventually cease their star formation. After presenting how galaxy images can be analysed to characterize their morphologies, I will describe how the properties of bulges and disks vary along the Hubble sequence, and how galaxies of diverse morphologies are distributed across the colour-mass diagram, as well as across various key scaling relations. This will highlight how morphological transformations, and especially the growth of the bulge, can be connected to their star-forming history. Then, I will discuss how the capabilities of ESA’s latest space telescope Euclid, launched in July 2023, can be leveraged to retrace the history of the Hubble sequence, and I will present the first strides made in that avenue of research, through the build-up of an unprecedented morphological catalogue, and the first analyses of galaxy morphology across cosmic times and along the Cosmic Web.

CRAL@ENS Seminar Dorottya Szécsi
2025-05-16 11:00 - 12:00 | ENS

CRAL@Obs seminar by Hakim Atek (IAP)
2025-04-25 11:00 - 12:00 | Observatoire

CRAL@ENS Seminar by Philipp Girichidis (Universität Heidelberg)
2025-04-18 11:00 - 12:00 | ENS
Cosmic rays in the interstellar medium
Cosmic rays (CRs) are high energy particles with with an energy density comparable to the other energy densities in the interstellar medium (ISM) and very distinct cooling and transport processes. The low cooling efficiency of CRs allows them to travel long distances from their location of acceleration. As a result, they have an impact on star formation at small scales and high densities. Furthermore, the pressure exerted by CRs fills the galaxy, enabling the launching of galactic-scale outflows. These outflows have a significant influence on galactic evolution.
In this review, I will focus on the dynamical impact of CRs, demonstrating how they shape the ISM and contribute to the launching of galactic outflows. There are significant uncertainties in our understanding of CR astrophysics, particularly regarding the speed at which they are transported and the efficiency with which they lose energy. I will highlight the efforts to address these uncertainties by incorporating CRs into hydrodynamical simulations. These simulations aim to accurately model their evolution from non-relativistic to relativistic regimes, while also considering the full magneto-hydrodynamical evolution. By doing so, this improves the accuracy of the CR-driven dynamics and allows us to generate synthetic observables such as gamma rays and synchrotron radiation, which provide valuable insights into the transport and cooling properties of CRs.

CRAL@Obs seminar by Hannah Ubler (MPE)
2025-04-11 11:00 - 12:00 | Observatoire
Observational constraints on massive black holes in the first billion years
One of the most surprising results coming out of the first years of science operations with JWST is the unexpectedly high abundance of actively accreting black holes in the early Universe. Compared to the local population, many of these early black holes appear to differ in various aspects, such as their relation to their host galaxies or their multi-wavelength properties. These observational findings challenge our understanding of the past evolution of present-day supermassive black holes, and provide new ways to constrain theoretical models of black hole formation and growth. I will give an overview of recent observational results on massive black holes in the first few billion years, driven by the unprecedented capabilities of JWST to explore cosmic dawn, and with a focus on results from the NIRSpec GTO surveys JADES and GA-NIFS, and the GO Large Program BlackTHUNDER.

CRAL@Obs colloquium by Joop Schaye (Leiden)
2025-04-04 11:00 - 12:00 | Observatoire

CRAL@Obs seminar by Michele Fumagalli (University of Milano Bicocca)
2025-03-28 11:00 - 12:00 | Observatoire

CRAL@Obs seminar by Lucie Leboulleux (IPAG)
2025-03-21 11:00 - 12:00 | Observatoire

CRAL@Obs seminar by Nicolas Laporte (LAM)
2025-03-07 08:00 - 17:00 | Observatoire

CRAL@Obs seminar by Guillaume Bourdarot (MPE)
2025-02-28 11:00 - 12:00 | Observatoire

CRAL@Obs seminar by Valentina D'Odorico (INAF)
2025-02-21 11:00 - 12:00 | Observatoire
Understanding metal enrichment in the early Universe through quasar spectroscopy —

Chemical elements heavier than helium – commonly referred to as “metals” in astrophysics – are synthesized by stars and distributed in the surrounding medium through various feedback mechanisms. This process plays a pivotal role in the baryon cycle of galaxies and is crucial for understanding galaxy evolution. In this seminar, I will provide an overview of how quasar absorption spectroscopy has been instrumental in studying the properties of metals within the diffuse circumgalactic and intergalactic media over the past two decades. I will particularly focus on observations spanning the timescale between the epoch of Reionization and the so-called cosmic noon at the peak of star formation, using state-of-the-art data like the spectra of z~6 quasars from the XQR-30 survey. Additionally, I will discuss the exciting challenges in the study of high redshift metal enrichment and how next-generation extremely large telescopes will help us address these issues.

CRAL@Obs seminar by Joe Lewis (IAP)
2025-02-14 11:00 - 12:00 | Observatoire

CRAL@Obs seminar by Nathan Hara (LAM)
2025-02-07 11:00 - 12:00 | Observatoire
Detecting Earth-analogs with radial velocities and photometry
Abstract:
Understanding how planets form and searching for life beyond Earth are two key goals in exoplanet research. To make progress on both fronts, it is crucial to discover Earth analogs. However, detecting them is a major challenge, as their faint signals are difficult to disentangle from astrophysical and instrumental noise. In this talk, I will present new data analysis techniques that significantly enhance the detectability of Earth analogs, and discuss the recent discovery of HD 20794 c, a 5.8M🜨 minimum mass crossing the habitable zone of a nearby star. Additionally, I will show that the methods developed for exoplanets can benefit other areas of astronomy—in particular how new stellar variability models open a new way to perform Doppler imaging.

CRAL@ENS internal seminar by Armand Leclerc
2025-01-31 11:00 - 12:00 | ENS
New tools from wave topology for asteroseismology
Asteroseismology studies waves in stars and uses them to infer what their internal structure is. Today, hundreds of stars of every kind show oscillations which allowed us to deduce their mass, radius, rotation rate, width of convective zone, etc. Wave topology proposes a new light under which to look at the wave equation, in order to obtain theoretical characterization of large-scale oscillation modes. This framework comes from quantum physics, and complement the classical theories of stellar oscillations. I will present its concepts, and show two new results obtained by it : a characterization of the fundamental mode n=0 at long horizontal wavelength, and a correction term for low n modes at short horizontal wavelength. Both these predictions are confirmed in the case of the Sun, opening perspectives for analytical low n modes seismology in other stars.
Where?
ENS-Monod – Amphi F

CRAL@Obs seminar by Nicolas Dobigeon (IRIT)
2025-01-21 11:00 | Observatoire
Multiband image fusion under spectrally varying spatial blurs — Application to JWST data
Multiband image fusion has been extensively studied in the literature of Earth observation. This task aims at recovering a multiband image of high spatial and high spectral resolutions from complementary measurements of lower spatial or spectral resolutions. While several methods developed in this context are able to fuse multiband images efficiently, they are not suitable to handle astronomical images, in particular in the context of observations made by the James Webb Space Telescope (JWST). Beyond the high dimensionality of the data, one major challenge lies in the operators associated with spatial blurs that should be considered as spectrally varying. This crucial issue significantly increases the complexity of the forward models and makes the state-of-the-art fusion methods inoperative. The main contributions reported in this talk tackle both challenges: a new framework is proposed to fuse large-scale astronomical data efficiently while taking into account the specificities of astronomical imaging.

CRAL@Obs PhD Defense by Théo Santos (CRAL, LGL)
2025-01-20 14:00 - 17:00 | Observatoire
Generative neural networks for parametrization in inverse methods: applications in astrophysics and geophysics
In physical science, it is common to study the characteristics of an object that are not directly available but which certain effects within the physical system are observable. The observations can then be used to deduce the properties of the object of interest: the approach known as “inverse” focuses on such problems. An essential question is the parametrization: how to properly define the parameters that define the object to study? The choice is crucial, as it determines the formulation of the inverse problem and therefore affects its difficulty to solve.
In recent years, machine learning, particularly generative neural networks, has offered a promising opportunity for addressing parametrization. These generative networks can generate, at will, objects similar to training objects, from only a few input parameters. A generative network can thus be trained on a set of possible objects and then used for parametrization. This approach presents several advantages: it is computationally efficient, it attenuates the irregularities between the parameters defining the object and the observations, it constrains the problem to a class of models similar to those in the training set, and it requires few degrees of freedom.
This thesis is devoted to the methodological analysis of this parametrization. In particular, I propose various tools to ensure the optimal training of a generator for this parametrization. I highlight the benefits, limitations, and specific considerations of this approach. To illustrate this methodological study, I rely on a toy problem consisting of recovering a clear image of a ring from a blurred and noisy version. I use it to illustrate the various tools and issues related to parametrization, which I apply within sampling and optimization inverse methods. Subsequently, I apply this approach to a geophysical problem and an astrophysical one to demonstrate its relevance in these domains. In geophysics, I apply this parametrization to a “downscaling” problem: the objective is to use a smooth image of the Earth’s mantle to derive a finer image containing small-scale details. To achieve this, I train the generative network on models created through geodynamic simulations; the network acts as an approximation of the simulation, more actionable for inverse problems. This application, performed on synthetic data, demonstrates the potential of this parametrization for future applications with real data. In astrophysics, I apply this approach to the generation of point spread functions (PSF). In astrophysical imaging, the PSF is an object, characteristic of the optical system, that defines how an image is blurred; estimating it accurately allows for efficient deblurring of an image. I thus parameterize the PSFs with a generative network, making the generator as an useful tool in inverse problems involving the PSF. For this case, the parametrization is performed on real PSF images, enabling to explore specific issues related to using real data.
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The presentation will be done in French.

CRAL@Obs seminar by Antoine Rocher (EPFL)
2024-12-20 11:00 - 12:00 | Observatoire
Galaxy-halo connection in cosmology with wide spectroscopic surveys —

Since the early 2000s, wide spectroscopic surveys, in particular multi-fibre experiments, have provided significant insights into the three-dimensional distribution of galaxies and quasars in the Universe, by constraining cosmological parameters through precise measurements of the baryon acoustic oscillation (BAO) scale and the non-linear growth rate of structure from redshift space distortions. Current and upcoming stage IV spectroscopic surveys, such as the Dark Energy Spectroscopic Instrument (DESI) or the 4-metre Multi-Object Spectrograph Telescope (4MOST) aims to reach sub-percent precision on cosmological parameters over a broad redshift range of 0.1 < z < 3.5.
In this talk, I will give an overview of the DESI survey and its scientific goals. In particular, I will focus on the galaxy-halo connection, i.e. how galaxies are connected to the underlying matter field. I will present results obtained using the early data release of DESI to constrain the connection between DESI galaxies (luminous red galaxies (LRGs) and Emission Line Galaxies (ELGs)) using the halo occupation distribution (HOD) framework, an empirical approach to link galaxies and dark matter halos in N-body simulations.
