Description du sujet de thèse

The evolution of galaxies and star formation are largely controlled by physical and chemical processes directly related to the interstellar medium (ISM). The ISM is an essential component of galaxies, subject to multiple heating mechanisms - ionization by UV photons, X-rays or cosmic rays, photoelectric effect on dust grains, shock waves, etc. - and the source of new stellar generations in the coldest clouds. An intense debate has arisen concerning the presence of super-Eddington stellar-mass black holes or intermediate-mass black holes in very low-metallicity galaxies. The implications are numerous as these sources may impact the interstellar medium and the star-formation history, and they may be at the origin of supermassive black holes through coalescence. This information is crucial for interpreting galaxy spectra at very high redshift with JWST.

The interstellar medium is rich with signatures, and these will be used to identify and constrain various physical processes at work. The thesis work will build on previous studies that focused in turn on photoionization by massive stars, photoionization by compact objects, and finally ionization by cosmic rays. Specifically, the thesis will explore the relative importance of interstellar shocks while improving the treatment of mechanisms already studied in our team. These models will be applied to a sample of nearby galaxies observed with various instruments, including JWST.

The overall objective of the ANR speXion (project managed by AIM, the University of Geneva and the Institut d'Astrophysique de Paris) is to conduct a coherent study of extremely metal-poor galaxies to understand the radiation field dominating these galaxies up to X-rays, characterize the nature and influence of energetic sources and associated feedback processes, and understand the contribution of these galaxies to cosmic reionization. The core of the thesis work will consist in exploring shock models and integrating them into the statistical framework for galaxy modeling, MULTIGRIS, developed locally and using a Bayesian probabilistic approach. The codes already exist, and it will be possible to contribute to the development of the MULTIGRIS code in particular. We will study spectroscopic and photometric tracers of ISM heating and cooling in a large sample of galaxies, in order to isolate the influence of different parameters (metallicity, presence of super star clusters, ultra-luminous X-ray sources, shocks...). JWST observations have been obtained by our team and will continue during the thesis, in addition to ground-based observations.

Contexte de travail

The thesis work will take place at AIM, which is the CNRS joint research unit associated with the Astrophysics Department of CEA, Saclay, located 30 km (40 minutes) from the center of Paris. Including all personnel, AIM comprises 200 people and offers a dynamic environment addressing various fields of astrophysics, from cosmology to exoplanetary systems.

The doctorate will be prepared within the framework of the Université Paris-Saclay, which will be the university of enrollment, and the Doctoral School of Astronomy and Astrophysics of Île-de-France (ED127).