CRAL

CRAL@Obs internal seminar by Louis Quilley
2025-05-23 11:00 - 12:00 (Europe/Paris) | 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
TBD

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.

CRAL@ENS seminar by Ugo Lebreuilly (CEA Saclay)
2024-12-13 11:00 - 12:00 | ENS

CRAL@Obs seminar by John Chisholm (UT Austin)
2024-11-29 11:00 - 12:00 | Observatoire
JWST’s GLIMPSE: A view of the faintest galaxies in the first billion years of cosmic time —

Faint high-redshift galaxies in the early universe constrain the properties of dark matter, the early build-up of stellar mass, and which sources reionized the early universe. The first two years of JWST observations have revealed a wealth of constraints on modestly bright galaxies in the first billion years of cosmic time, but the faintest population has remained under-probed. JWST’s Glimpse is a Cycle 2 large program that pairs up to 39 hour integrations with the natural lensing of the massive foreground cluster Abell S1063. These observations probe a new regime of intrinsically faint galaxies (down to MUV of -10 or observed magnitudes of 34) in the early universe. With seven wide and two medium bands, we find a new class of galaxy candidates within the first 200 Myr of cosmic time that must have formed extremely efficiently. We also constrain the number of very faint galaxies in Cosmic Dawn, the star formation histories of early galaxies, and explore how the faintest galaxies contributed to reionization. In this talk I will highlight our preliminary effort to reveal the population of the faintest early galaxies and their impact on the evolution of the universe.

Les Conférences, Association française d'astronomie
2024-11-09 08:00 - 2024-11-11 17:00
Plus de trente conférenciers qui font l’actualité astronomique et spatiale vous convient à un voyage extraordinaire jusqu’aux frontières de la connaissance. Il y est question de cosmologie, d’espace, de planétologie, d’histoire des sciences, de physique fondamentale, de biologie… Connus pour leur aisance, leur propension à nous faire vivre les découvertes, leur aptitude à expliquer simplement des concepts difficiles… ou tout simplement leur passion, les orateurs et oratrices des Rencontres sont des scientifiques, des spécialistes.

Séminaire de Laurent Mugnier (ONERA) à l'Observatoire
2024-10-22 15:30 - 17:00
Focal-plane wavefront sensing and applications : from Earth observation to astronomical imaging, laser focusing and exoplanet detection
