Séminaires LERMA

Observatoire de Paris - PSL - CNRS

Archives 2012-2019
Archives 2001-2011
Email : jean-francois.lestrade@obspm.fr







résumé : Tba



mercredi 19 juillet 2023, 14H

Salle J-F Denisse (ex-Atelier), Observatoire de Paris, VIDEO

Galaxy clusters in MOND: the case of ultra diffuse galaxies in the Coma cluster

Srikanth T. NAGESH, Observatoire de Strasbourg

résumé : Ultra diffuse galaxies (UDG), low-surface brightness objects with large effective radii, inside galaxy clusters have very low internal gravity which renders them ideal candidates for testing the Modified Newtonian Dynamics (MOND) paradigm as a possible alternative to dark matter. Freundlich et. al. (2022) studied the velocity dispersions of several UDGs in the Coma cluster and compared them with the predictions of MOND. While the agreement would have been excellent if these galaxies were isolated, there seems to be an apparent disagreement because of the so-called `` external field effect '' (EFE) related to the gravitational influence of the cluster. The authors have proposed several scenarios that could possibly explain this discrepancy. Building on this work, I will be presenting results of numerical simulations of UDGs orbiting around the Coma cluster, in MOND. We look at some of their dynamical properties and compare them with observations and subsequently try to explain whether the UDGs are within the realm of reconciliation or beyond.


Vendredi 16 juin 2023, 14h00

Salle de l'Atelier, Observatoire de Paris, VIDEO

Toward more realistic Star Forming galaxy model


résumé : Most of the models build to compute the nebular emission of Star Forming Galaxies are obtained using very simple descriptions of the gas and its photoionizing source. In this talk I will present a new method that takes into account a more realistic description of the galaxies, in terms of sums of hundreds or thousands of individual HII regions. The emission of each region is very quickly obtained by the use of machine learning algorithms (Artificial Neural Networks). I will discuss some details of the modeling, and present preliminary results, involving methods to determine abundances of the galaxies. Comparisons to observed surveys are also discussed.


Vendredi 14 avril 2023, 14h00

Salle de l'Atelier, Observatoire de Paris, VIDEO

What is the origin of the different kinematic morphologies of early-type galaxies ?

Michal BILEK, LERMA/Observatoire de Paris

résumé : 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, or 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 mergers more than 10 Gyr ago.
Based on https://arxiv.org/abs/2210.02478 (accepted for publication in A&A)


lundi 27 mars 2023, 14h00

Salle du Levant, Observatoire de Paris, VIDEO

Learning about Star Formation in Galaxies using the UVIT and MUSE

Mousumi DAS, Indian Institute of Astrophysics, Bangalore

résumé : Star formation is one of the dominant processes driving galaxy evolution. Galaxies with cold gas are generally found to host star formation in their inner disks, but the intensity of star formation increases dramatically when galaxies come close. UV observations are one of the best ways to study the distribution and intensity of star formation in galaxies. In this talk I will present an overview of our UV imaging telescope (UVIT) and MUSE studies of star formation in nearby galaxies. This includes several studies on interacting and merging galaxies as well as dual AGN host galaxies. I will also describe the unusual star formation in extended UV disk galaxies and briefly summarise our recent work on star formation in dwarf galaxies.


Vendredi 3 mars 2023, 14h00

Salle de l'Atelier, Observatoire de Paris, VIDEO

On the enigmatic "beasts" - giant low surface brightness galaxies

Anna SABUROVA, on leave from Sternberg Astronomical Institute

résumé : Giant low surface brightness galaxies (gLSBGs) have the largest discs in the Universe with the radii up to 130 kpc. The formation of such enormous discs is a stress-test for the hierarchical galaxy formation paradigm and without clarifying it we cannot paint a coherent picture of galaxy evolution. In the talk I will give the answers to the following questions. How rare are gLSBGs? What are the formation scenarios of gLSBGs? And how does it all correspond to the results of modern cosmological simulations? These answers are based on both in-depth study of 8 gLSBGs, including the results of our deep spectroscopic and photometric observations, HI data collected in the framework of our observing programs and complemented by archival datasets. Finally, we used deep optical images from HSC Subaru Strategic Program and publicly available redshift catalogs, estimated the volume density of gLSBGs in the local Universe and compared it to state-of-the-art numerical simulations.


Vendredi 10 février 2023, 14h00

Salle de l'Atelier, Observatoire de Paris, VIDEO

"Challenges for new discoveries from cosmic microwave background experiments towards the age of inflation and beyond"


résumé : In the seminar I will discuss the current challenges for an unbiased detection of the CMB polarization signal with the accuracy required to probe physics beyond the current standard model of cosmology. Specifically, I will present a study project developed at the LPENS and LERMA laboratories in Paris to face this challenge.
Over the past two decades, observations of temperature fluctuations in the cosmic microwave background (CMB) have played a fundamental role in defining the current standard cosmological model, providing insights into the origin of structure, the density of baryons, dark matter, dark energy and the global properties of spacetime. Today, one of the biggest unanswered questions concerns the mechanism that created the small primordial fluctuations that fuelled the observed anisotropies of the CMB and that eventually grew into stars and galaxies. The most widely accepted idea is that of 'cosmic inflation', which predicts an almost exponential expansion of the universe at its origins.
The curl-like pattern of the CMB polarisation, the so-called B-modes, is the observable signature of the existence of the primordial gravitational waves predicted by the inflation theory. The detection of this very faint signal could therefore probe the existence of an epoch of inflation. This aim has motivated an enormous technological effort to achieve unprecedented levels of instrument sensitivity. Today, however, the limiting factors are related more to uncertainty (astrophysical and instrumental) than to statistical uncertainty. Two of the main problems concern: i) the absolute calibration of the polarisation angle and ii) the removal of the CMB foreground emission. These systematics are partially mitigated by data analysis and model-dependent assumptions, but the lack of an efficient and independent method to accurately calibrate CMB experiments could deny the possibility of potential new discoveries in the field.
In this context, recent results (Ritacco+2023) have shown that important information is still missing to correctly model the Spectral Energy Distribution of the dust foreground in the E- and B-mode polarization signals, which could significantly distort the CMB signal. I will present these new findings enclosed in a recent release of Planck HFI data together with the state-of-the-art concerning the absolute calibration of CMB polarization experiments through the well known supernova remnant, the Crab nebula.


MERCREDI 14 Décembre 2022, 14h00

Salle de l'Atelier, Observatoire de Paris, VIDEO

"Molecular gas in HI-absorption-selected galaxies at z~2"

Balpreet KAUR, National Centre for Radio Astrophysics, Pune, India

résumé : The strongest HI absorbers, the damped Lyman-alpha absorbers (DLAs), in QSO spectra indicate the presence of a gas-rich galaxy close to, or along, the QSO sightline. Identifying the HI-selected galaxies associated with DLAs provides a unique opportunity to connect the properties of high-z galaxies to those of their circumgalactic mediums, as well as to identify and characterize galaxies without a luminosity bias. We have hence been using NOEMA, the JVLA, and ALMA to carry out searches for redshifted CO emission from galaxies associated with DLAs at z>~2. Combining our results with ALMA studies from the literature, we find that the highest-metallicity DLAs tend to be associated with galaxies with very high molecular gas masses, > 5~10^10 solar masses, while the galaxies associated with lower-metallicity DLAs are not detected in their CO emission. We have also used the JVLA to detect, for the first time, CO(1-0) emission from high-z HI-selected galaxies, allowing us to measure the excitation of the mid-J CO rotational levels. We find that all four HI-selected galaxies at z>2 with CO(1-0) studies show high excitation of the mid-J CO rotational levels, with near-thermal excitation found in the two galaxies with CO(1-0) detections. The mid-J excitation in the HI-selected galaxies is consistent with that in main-sequence and sub-mm galaxies at z>2, but significantly higher than that in main-sequence galaxies at z<2. The CO-detected galaxies are faint in our HST NUV images, indicating that they are highly obscured objects.


JEUDI 7 Juillet 2022, 14h00

Salle de l'Atelier, Observatoire de Paris, VIDEO

"Cosmological QUOKKAS: A new way of measuring cosmological distances"

Jeffrey HODGSON, Sejong University, Seoul

résumé : Recently there has been substantial debate around the reliability of cosmological distance measures. Potential sources of systematic error could be influencing the interpretation of results. For this reason, there is a great need for a new and independent distance measure. In this presentation, I will present the 'standard speed-gun' method of measuring distances to blazars which can be seen from 0 < z < 6. The core assumption of the method is that the variability seen in blazars is constrained by the speed of light. This then allows us to calibrate a standard ruler which is then compared with the apparent size; measured with VLBI. We applied this method to the famous nearby source 3C 84 and derived a measurement of the Hubble Constant. This technique has several advantages over other distance measures such as Type Ia supernovae but is currently limited by the sensitivity and cadence of existing VLBI arrays. The Cosmological QUOKKAS program will use the KVN with the Mopra telescope in Australia to measure distances to AGN over a large redshift range. The initial program is somewhat limited, but new arrays such as SKA-VLBI and the ngVLA could potentially allow distance measurements to tens of thousands of sources or more.


JEUDI 30 Juin 2022, 14h00

Salle de l'Atelier, Observatoire de Paris, VIDEO

"Constraining the properties of resolved gas and dust disks of local spiral galaxies"

Thomas LIZEE, Observatoire de Strasbourg

résumé : Milky Way observations have provided insight into the scaling relations of molecular clouds and their ability to form stars. However, these relations cannot be established in nearby galaxies due to the limited spatial resolution of available observations. Starting from the multiphase, multiscale analytical model of Vollmer et al. 2017, whose goal is to describe galaxies as clumpy turbulent accretion disks, we improved the model and applied it to a sample of 17 local spiral galaxies. We generated radial profiles of large-scale quantities (SFR, stars, total gas) as well as molecular line emissions of different species (CO, HCN, HCO+) and compared them to multi-wavelength observations. The model is then able to predict key properties of the ISM such as the Toomre parameter Q, the gas velocity dispersion, the characteristic timescales of molecular clouds (free fall, molecular gas formation and turbulent times) as well as the CO-to-H2 and HCN-to-H2(dense) conversion factors. We conclude our study by reproducing the radial profiles of a Virgo cluster galaxy, NGC 4654, affected by both ram pressure stripping and gravitational interactions to better understand how the ISM properties reacts to such perturbations.


JEUDI 16 Juin 2022, 14h00

Salle de l'Atelier, Observatoire de Paris, VIDEO (voice for first 15 minutes only)

"The carbon cycle emission as a diagnostic tool for the interstellar medium state"

Thomas BISBAS, University of Cologne

résumé : Carbon is one of the most abundant elements in the Universe and present in both local and distant galaxies. It can be found in three forms, known as the "carbon cycle": ionized (C+), atomic (C), and molecular in the form of carbon monoxide (CO). Its chemical state is determined by the environmental parameters of the interstellar medium. Consequently, we can use the emission lines of the carbon cycle to estimate these environmental parameters, which ultimately shape the density distribution of the interstellar gas in galaxies. Through numerical modelling and comparison with observational data, I will discuss how each of the aforementioned line emissions can be used as a diagnostic of the chemical and dynamical state of the interstellar medium.


JEUDI 9 Juin 2022, 14h00

Salle de l'Atelier, Observatoire de Paris,

"Evidence for amino acids in the gas of the IC 348 star cluster of Perseus"

Susana IGLESIAS ROTH, Instituto de Astrofisica de Canarias

résumé : Amino acids are building-blocks of proteins, basic constituents of all organisms and essential to life on Earth. They are present in carbonaceous chondrite meteorites and comets, but their origin is still unknown. Formation of amino acids in the interstellar medium is posible via specific gas-phase reactions in dark clouds, however sensitive radiosearches at millimeter wavelengths have not revealed their existence yet. The mid-IR vibrational spectra of amino acids provides an alternative path for their identification. We present Spitzer spectroscopic observations in the star-forming region IC 348 of the Perseus Molecular Cloud showing evidence for mid-IR bands of H2, OH, H2O, CO2, C2H2, C4H2, HC5N, C2H6, C6H2, C6H6, PAHs, fullerenes C60 and C70 and emission lines consistent with the most intense laboratory bands of the three aromatic amino acids, tyrosine, phenylalanine and tryptophan and the aliphatic amino acids isoleucine and glycine. Estimates of column densities give values 10-100 times higher for isoleucine and glycine than for the aromatic amino acids as in some meteorites. The strongest bands of each amino acid are also found in the combined spectrum of >30 interstellar locations in diverse star-forming regions supporting the suggestion that amino acids are widely spread in interstellar space. Future mid-IR searches for proteinogenic amino acids in protostars, protoplanetary disks and in the interstellar medium will be key to establish an exogenous origin of meteoritic amino acids and to understand how the prebiotic conditions for life were set in the early Earth.


JEUDI 7 avril 2022, 14h00

Salle de l'Atelier, Observatoire de Paris,

"The Condor Array Telescope: Deep, Rapid-Cadence, Broad- and Narrow-Band Imaging"

Kenneth LANZETTA, University of New York, Stony Brook

résumé : The Condor Array Telescope or “Condor” is an “array telescope” consisting of six off-the-shelf 180 mm-diameter apochromatic refracting telescopes coupled with six off-the-shelf large-format CMOS camera all mounted onto a common mount. The telescope is specifically designed to (1) detect and identify galaxies and galaxy features of extremely low surface brightness, (2) monitor stars and other point sources at a very rapid cadence, and (3) study the gaseous signatures of star birth and star death. The telescope was deployed to a dark site in the US desert southwest near Animas, New Mexico last spring and was commissioned and calibrated over the course of last summer and autumn. In this lecture, I will describe the telescope, report the current status of the project, and discuss the observations that Condor has obtained over its first year of operation. I will specifically discuss the current state of Condor’s search for Earth-like planets in the habitable zones of white dwarfs.


JEUDI 24 mars 2022, 14h00

Salle de l'Atelier, Observatoire de Paris, VIDEO

"Environmental effects on galaxies"

Florence DURRET, IAP

résumé : The properties of galaxies are known to be modified by their environment, and this is particularly visible in galaxy clusters, where ram pressure stripping (RPS) by the hot intracluster medium may occur. I will present recent searches for RPS galaxies in clusters at intermediate redshifts (between 0.4 and 0.9) based on HST images, concentrating particularly on two clusters at z=0.5 for which a detailed study of their stellar properties (masses, ages, star formation rates) was possible.

Interestingly, RPS candidates with spectroscopic and with photometric redshifts have comparable stellar properties, suggesting that large samples of RPS galaxies could be gathered based on multi-band photometry only, a promising result in view of the very large imaging surveys planned in the coming years (DES, Euclid, LSST, etc.).

I will also discuss the properties of brightest cluster galaxies (BCGs), the influence of intracluster light on these galaxies, and the link of BCGs with the cosmic web.


JEUDI 24 fevrier 2022, 16h00, i.e. !!! SEIZE HEURES !!!


The LOFAR window on the multiphase and magnetized Galaxy: insights from the MUSICA project

Andrea BRACCO, Rudjer Bošković Institute, Zagreb & ENS

résumé : Radio observations are a unique window to explore the mystery of cosmic magnetism, from the Galaxy to cosmological ages. By tracing synchrotron emission below 200 MHz with unprecedented sensitivity, the LOFAR telescope is providing us with the most detailed image of the magnetized Milky Way that we have just started to study. In this talk I will present the progress of my MUSICA project. We have contributed to step forward in the understanding of the radio sky, highlighting with first statistical results the complex coupling of the multiphase interstellar gas with magnetic fields in our Galaxy. Our observational results and numerical models of the turbulent, multiphase, and magnetized Galaxy are preparing the ground for the interpretation of next-generation radio-telescopes such as NenuFAR, LOFAR2.0, and the Square kilometer Array.


JEUDI 17 fevrier 2022, 14h00

Visioconference VIDEO (first 3 minutes missing)

Cold gas and galaxy evolution in massive groups/clusters at high redshifts

Emanuele DADDI, CEA

résumé : According to theory expectations, massive halos at high redshifts should be efficiently fed with cold gas streams from the cosmic web, and display intense activity as a result. Still, after decades of simulation work on these processes, not much has been obtained from observations in support of this theory, as directly observing accretion is very difficult. In my talk I will describe ongoing efforts to systematically characterise the presence and nature of cold gas shining in Lya around massive structures at high redshifts, based on KCWI follow-up of known structures. Our best studied case, RO-1001 at z=2.91, reveals a filamentary structure highly suggestive of accreting gas. Based on our current sample of 9 targets, we investigate trends between hosting DM halo mass, integrated SFRs, AGN bolometric activity and redshift, as a function of Lya luminosity and predicted properties, trying to unveil possible patterns pointing to accretion.
We also concentrated on galaxy properties in these fields, seeking signatures that could relate to accretion. I will discuss the present evidence in the framework of current understanding of gas accreetion, outflows, feedback and growth of structures with their galaxy content.


JEUDI 3 fevrier 2022, 14h00


The role of hub-filament systems in the formation of low- to high-mass stars

Doris ARZOUMANIAN, NAOJ, Nagoya University, Japan

résumé : Recent observations have revealed the organization of the interstellar medium into filament networks. In molecular clouds, the densest filaments are now identified as the precise birthplaces of individual stars, while the hubs formed at their intersections are the location of stellar clusters and high-mass stars. To understand the star formation process, it is thus essential to describe the formation and evolution of filaments and hubs and their fragmentation into star forming cores. I will present new theoretical and observational studies indicating the formation of filamentary molecular clouds at the edge of expanding bubbles. In particular, I will show observations suggesting multiple compressions responsible in the formation and evolution of the filament networks. I will also discuss the role of filament coalescence and hub-filament systems in the formation process of stars from low to high masses.


JEUDI 20 janvier 2022, 14h00

Visioconférence VIDEO

Scattering transforms, application to astrophysics and cosmology


résumé : Thanks to their ability to efficiently extract non-Gaussian features, as well as allowing realisitic generative models to be built from a very small amount of data, scattering transforms can be used as an important lever for scientific goals in astrophysics and cosmology. In this talk, I will focus on two particular topics. First, the comparison between different observations and simulations of the interstellar medium, the underlying goal of which is to develop simulations capable of reproducing the nonlinear dynamics observed in the sky. Second, the separation of components in astrophysical observations thanks to their different non-Gaussian signatures, in particular the CMB/CIB and their respective galactic foregrounds.


LUNDI 6 décembre 2021, 14h00

Salle 304 T32-33 Jussieu/LERMA VIDEO

Universal PAH fragmentation: The search for common carbon ancestors

Helgi Rafn HRODMARSSON, Leiden Observatory

résumé : The chemical link between the diffuse interstellar medium and molecular clouds is not well characterized. This is in part because the century old mystery of the Diffuse Interstellar Bands (DIBs) is still unsolved, as it could hold the keys to unlocking which are the most important molecules capable of surviving in the diffuse interstellar medium for aeons. By studying the radiation-induced fragmentation of several PAH species of different symmetry and different sizes, this work shows that PAHs appear to follow very similar fragmentation pathways, giving rise to ionized magic number carbon clusters. These could be ideal candidates as products of PAH destruction in PDRs and other astrophysical regions that ungergo significant energetic processing. If this scenario is taken to the extreme, these clusters might survive in the diffuse interstellar medium for a long enough period to form the first chemical seeds of molecular clouds – becoming the common carbon ancestors of stars, planets, and life.


JEUDI 2 décembre 2021, 11h00 (!!)

Salle de l'Atelier, Observatoire de Paris VIDEO

Big data, Big responsabilities: Introduction to Maneage (Managing data lineage)

Mohammad AKHLAGHI, Instituto de Astrofísica de Canarias (IAC), Spain

Abstract. The increasing volume, diversity, and role of data in modern research has been very fruitful. However, these same factors, have also made it harder to describe (in sufficient detail) the processing behind a scientific result within the confines of a traditional paper. It is thus becoming harder and harder to reproduce results (i.e., critically review by coauthors, referees or larger community) that define scientific progress. In this talk, Maneage (MANaging data linEAGE) is introduced as a working solution to this problem. Maneage is (a template) that provides a framework to exactly reproduce a scientific analysis (from the input data and software, to the processing and creation of final report/paper/dataset. The necessary software are built (from the low-level C compiler and shell, to the higher-level science programs and all their dependencies) with the predefined configuration. The software are then run on the input data sets to produce the final result. The template will finally produce a "dynamic" PDF using LaTeX macros: any change in the analysis will automatically update the relevant parts of the PDF (for example numbers, tables or figures). A project defined in this template is fully managed and published in plain text and only consumes a few hundred kilo-bytes (unlike binary multi-gigabyte blobs like containers). It is thus easily to publish, for example on arXiv with the paper's LaTeX source or Software Heritage. Giving readers the ability to exactly study and reproduce the paper's results. It is also easily search-able, providing a treasure trove to extract metadata on the project (even after publication, and without the author's active involvement). This can be very valuable when implement widely (e.g., using machine learning on many project sources to define automatic workflows). But most importantly it will allow other scientists to independently study the details, verify in practice, and build incrementally on each others' work, without necessarily needing to run it.


MERCREDI 10 novembre 2021, 14h00

Salle de l'Atelier, Observatoire de Paris, VIDEO

Do we need a new cosmological model? Towards accurate precision cosmology with CLONES

Jenny SORCE, ENS Lyon

résumé : 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, small scale simulations mismatch detailed small scale observations. 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-induced biases are of the same order of magnitude. A precise mapping and modeling 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 studies. I will summarize the genesis of the initial conditions of such simulations, that I baptized CLONES (Constrained LOcal & Nesting Environment Simulations), as well as present a few results that promise to tremendously impact our capacity to evade local-induced biases that will matter in future analyses to reach accurate precision cosmology.


LUNDI 11 octobre 2021, 14h00

Salle de l'Atelier, Observatoire de Paris, VIDEO

Transforming gas-rich low-mass discy galaxies into ultra-diffuse galaxies by ram pressure


APC U. Paris, LERMA ObsPM, Sternberg Astronomical Institute Moscow State University

résumé : Faint extended elliptically-shaped ultra-diffuse galaxies (UDG) and slightly brighter and more compact dwarf elliptical (dE) and lenticular stellar systems are common in galaxy clusters. Their poorly constrained evolutionary paths can be studied by identifying young UDG and dE analogs populated with bright, massive stars. Using data mining we identified 11 such low-mass (between 2e8 and 2e9 Msun) galaxies with large half-light radii (between 2 and 5 kpc) and recently quenched star formation in the Coma and Abell 2147 galaxy clusters. All galaxies happen to have ram-pressure-stripped tails with signs of current or recent star formation. Deep spectroscopic observations revealed rotating stellar discs containing 70-95% dark matter by mass. A large fraction of the disc stars (10-60%) formed in intense star bursts 180-970 Myr ago, likely triggered by ram pressure. Observed global gradients of stellar age corroborate this scenario. Passive evolution in the next 10 Gyr will transform 9 of the 11 galaxies into UDGs. If we assume a constant rate of galaxy infall, 44±16% of the most luminous present-day UDGs in Coma must have formed via ram pressure stripping of discy progenitors.


Vendredi 23 avril 2021, 14h00

Visioconférence VIDEO

A Stellar Graveyard in the Core of a Globular Cluster,


résumé : The ubiquity of supermassive black holes in massive galaxies suggests the existence of intermediate-mass ones (IMBHs) in smaller systems. However, IMBHs are at best rare in dwarf galaxies and not convincingly seen in globular clusters. We embarked on a search for such an IMBH in a very nearby core-collapsed globular cluster, NGC 7397. For this we ran extensive mass-orbit modeling with our Bayesian MAMPOSSt-PM code that fits mass and velocity anisotropy models to the distribution of observed tracers in 4D projected phase space. We used a combination of proper motions from HST and Gaia, supplemented with redshifts from MUSE. We found very strong Bayesian evidence for an excess of unseen mass in the core of the cluster amounting to 1 to 2% of the cluster mass. But surprisingly, we found rather strong evidence that this excess mass is not point-like but has a size of roughly 3% of that of the cluster. Our conclusion is robust to our adopted surface density profile and on our modeling of the velocity anisotropy, as the data suggest isotropic orbits throughout the cluster. It is also robust to our use of one or two classes of Main Sequence stars (given the mass segregation in collisional systems such as clusters), as well as on our filtering for quality data. The expected mass segregation suggests that the excess mass is made of objects heavier than Main Sequence stars: white dwarfs, neutron stars and possibly stellar black holes, all of which lost their orbital energy by dynamical friction to end up in the cluster core. I will discuss the evidence for and against the possibility that most of the unseen mass in the center is in the form of such black holes, as well as the consequences of this intriguing possibility.


Vendredi 19 mars 2021, 14h00

Visioconférence VIDEO

Millimeter rotational lines as powerful diagnostics of the physical conditions inside a Giant Molecular Cloud - The Orion B case

Jérôme PETY, IRAM/Observatoire de Paris-LERMA

résumé : Molecules have long been thought to be versatile tracers of cold neutral media in the universe, from high-redshift galaxies to star forming regions and proto-planetary disks, because their internal degrees of freedom are controlled by physical conditions in their environments. However, the promise that molecular emission has a strong predictive power of the physical and chemical state of the interstellar medium is still hampered by the incomplete understanding of the complex physical and chemical structure of the interstellar gas, and its dynamical evolution.

In this talk, I will present The ORION-B project (Outstanding Radio-Imaging of OrioN-B), a Large Program of the IRAM 30m telescope, and discuss three recent results obtained thanks to machine learning techniques: 1) How to improve the quantitative estimate of the H2 column density based on the molecular line intensities (Gratier et al. 2021);

2) How to estimate a reference precision on the column densities, excitation temperatures, centroid velocities and velocity dispersions of the three main CO isotopologues, 12CO, 13CO, C18O, with the help of the Cramer Rao Bound (CRB) technique (Roueff et al. 2021);

3) How to identify the best tracers of the ionization fraction among hundreds of species included in models (Bron et al. 2021). I will discuss the astrophysical consequences.

References : * Bron et al. 2021, A&A, 645, 28B, "Tracers of the ionization fraction in dense and translucent gas. I. Automated exploitation of massive astrochemical model grids" * Gratier et al. 2021, A&A, 645, 27G, "Quantitative inference of the H2 column densities from 3 mm molecular emission: case study towards Orion B" * Roueff et al. 2021, A&A, 645, 26R, "C18O, 13CO, and 12CO abundances and excitation temperatures in the Orion B molecular cloud. Analysis of the achievable precision in modeling spectral lines within the approximation of the local thermodynamic equilibrium".


Vendredi 5 mars 2021, 14h00

Visioconférence VIDEO

Laboratory astrophysics studies of VUV and X-ray induced photodesorption from interstellar ice analogues

Mathieu BERTIN, LERMA-Jussieu-PSL

résumé : The recent advances in space and ground based telescopes (ALMA, NOEMA…) have allowed the detection of more and more molecules in the gas phase in the coldest regions of the interstellar medium - ISM (star-forming regions, protoplanetary disks…). The puzzling detection of these gaseous species, including small organic molecules, in media where the temperature is very low (~ 10-100 K), is currently a major and still open question, directly linked to the astrochemical richness. Most of the observed molecules are indeed expected to either directly form or accrete on the surface of dust grains, and cannot thermally desorb in the regions where they are detected. Their observation requires thus non-thermal desorption processes, among which the desorption induced by UV or X-ray photons – so-called photodesorption – is a promising candidate. However, its role still needs to be clarified, especially in the case of the desorption of small organics molecules for which both the quantitative yields and the underlying mechanisms are lacking.

I will present the outcomes of recent laboratory astrophysics studies base on the use of the monochromatic and tunable synchrotron radiation, dedicated to understand and quantify the photodesorption processes in both the vacuum UV (7-13,6 eV) and soft X-rays (500 – 1500 eV) energy ranges. The role played by the photon energy and of the molecular ice composition on the desorption yields will be highlighted, and a special focus will be made on the case of photodesorption of complex organics molecules.


Vendredi 26 février 2021, 14h

Visioconférence VIDEO

Morphological Transformations and Quenching in Galaxies : from Simulations to Observations.

Marc HUERTAS-COMPANY, LERMA-Observatoire de Paris

Résumé : The origins of the relation between galaxy structure and star formation is still debated. I will discuss recent efforts to advance in our understanding of how massive galaxies change their morphology and quench from z~3. Using several state of the art deep learning techniques, we try to link hydrodynamic cosmological simulations with observations from deep surveys to constrain the physical conditions and evolutionary tracks of galaxies.


Vendredi 22 janvier 2021, 14h00

Visioconférence VIDEO

Dark Matter Halo Response to Baryons

Jonathan FREUNDLICH, Observatoire Astronomique de Strasbourg

Résumé : While cold dark matter numerical simulations predict steep, `cuspy' density profiles for dark matter halos, observations favour shallower `cores'. The introduction of baryonic physics alleviates this discrepancy, notably as feedback-driven outflow episodes contribute to expand the dark matter distribution for stellar masses between 10^7 and 10^10 Msun. I will first present a parametrization of dark matter halo density profiles with variable inner slope and concentration that enables to describe the variety of halo responses to baryons and has analytic expressions for the gravitational potential, the velocity dispersion, and lensing properties. This parametrization provides a useful tool to study the evolution of dark matter haloes, to model rotation curves of galaxies and gravitational lenses, and to be implemented in semi-analytical models of galaxy evolution. I will then present two theoretical models describing core formation in dark matter haloes. In the first one, sudden bulk outflows induced by stellar feedback reorganise the halo mass distribution while it relaxes to a new equilibrium. In the second one, small stochastic density fluctuations induce kicks to collisionless particles that progressively deviate them from their orbits. Both models are tested against numerical simulations and provide a simple understanding of the transition from cusps to cores by feedback-driven outflows.


Vendredi 11 décembre 2020, 14h00

Visioconférence VIDEO

Investigating the physical processes driving the evolution of baryons in local and high-redshift low-metallicity galaxies

Ambra NANNI, Laboratoire d'Astrophysique de Marseille & National Centre of Nuclear Research, Warsaw

Résumé : 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.


Vendredi 4 décembre 2020, 14h00

Visioconférence VIDEO

Simulating galaxies at high resolution in their cosmological context with NewHorizon: methods and some key results on galaxy properties and their morphology

Yohan DUBOIS, Institut d'Astrophysique de Paris

Résumé : Hydrodynamical cosmological simulations are increasing their level of realism by considering more physical processes, having more resolution or larger statistics. However, one usually has toeither sacrifice the statistical power of such simulations or the resolution reach within galaxies. I will introduce the NewHorizon project where a zoom-in region of ~(16 Mpc)^3, larger than a standard zoom-in region around a single halo, embedded in a larger box is simulated at high resolution. A resolution of up to 34 pc, typical of individual zoom-in state-of-the-art resimulated halos is reached within galaxies, allowing the simulation to capture the multi-phase nature of the interstellar medium and the clumpy nature of the star formation process in galaxies. I will present and discuss several key fundamental properties of galaxies and of their black holes. Due to its exquisite spatial resolution, NewHorizon captures the inefficient process of star formation in galaxies, which evolve over time from being more turbulent, gas-rich and star-bursting at high redshift. These high redshift galaxies are also more compact, and are more elliptical, disturbed and clumpier until the level of internal gas turbulence decays enough to allow for the formation of stable rotating discs. I will show the origin and persistence of the thin and thick disc components, and explain why the settling of discs ``magically’’ occurs at around a stellar mass of 1e10 Msun.


Vendredi 27 novembre 2020, 14h00

visioconférence SLIDES

La phosphine sur Vénus : Une histoire brève qui en dit long

Thérèse ENCRENAZ, LESIA, Observatoire de Paris

résumé : Le 14 septembre 2020, une nouvelle fait sensation dans le monde des astronomes… et au-delà. A partir de données obtenues avec le JCMT et ALMA, une équipe internationale pilotée par Jane Greaves (Université de Cardiff, UK) annonce la détection de la phosphine (PH3) dans l’atmosphère de Vénus et évoque la possibilité d’une forme de vie sur la planète. Plusieurs articles sont publiés ou soumis dans la foulée, étudiant les qualités éventuelles de la phosphine en tant que biomarqueur ou évoquant un développement possible de la vie dans les nuages de Vénus, et la presse internationale en fait grand cas. De notre côté, au LESIA, nous menons depuis huit ans une campagne d’observations de Vénus en infrarouge, avec le télescope IRTF de Maunakea, pour étudier les variations de deux molécules mineures importantes pour la climatologie de Vénus, SO2 et H2O. En mars 2020, nous avons été contactés par l’équipe de J. Greaves pour rechercher la phosphine dans l’infrarouge avec l’instrument que nous utilisons. Les observations n’auront pas lieu à cause de la crise sanitaire. Cependant, nous avons recherché la signature de PH3 dans d’anciennes données infrarouges prises avec le même instrument, et nous en avons déduit pour l’abondance de PH3 une limite supérieure très contraignante, quatre fois plus faible que la valeur annoncée par Jane Greaves et ses collègues. Par ailleurs, le traitement des données ALMA par son équipe fait l’objet de nombreuses réserves ou critiques de la part des radioastronomes. Il est donc vraisemblable que l’engouement des média pour la phosphine de Vénus va retomber prochainement…