Séminaires LERMA

Observatoire de Paris - PSL - CNRS

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

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 q! uantitative 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, sudde! n 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 o! ver 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 SLIDE

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…