GEPI

Subject : The releases of the first and second Gaia catalogues have driven a profound change in our definition and comprehension of all Galactic stellar populations, as they were known until recently. The Galactic disc appears as old as the Galaxy itself, and a not negligible fraction of stars at metallicities usually associated to the halo population are indeed on disc-like orbits. The study of its chemo-kinematic properties has allowed our group to recently discover the signatures of the last significant merger experienced by our Galaxy, between 9 and 11 Gyr ago, and to show the imprint this merger left on the kinematics of its stars. The halo of our Galaxy, at least inside ∼20 kpc from the Galactic centre (the so-called inner halo), seems now made of two main populations : accreted stars from a massive (or two ?) satellite(s), and the early Galactic disc, kinematically heated at the time this(these) accretion(s) occurred. These results have a number of consequences on our understanding of Galactic stellar populations, as they have been described and interpreted until now, in particular at the interface between the disc and the halo.

With this internship, we aim at going deeper in the study of the Galactic halo, its formation, and evolutionary link with the Galactic disc, by coupling the exquisite Gaia astrometry to complementary spectroscopic surveys, like APOGEE and Galah, which provide radial velocities and chemical abundances for hundreds of thousand stars in our Galaxy. By making use of these data, we aim at :
– constraining the number of massive accretions experienced by the Galaxy in its early phases of evolution. The intent here is to look for the existence of accreted stars from satellites which, at the time of their accretion, had a total mass equal to about one-tenth of the mass of the Milky Way
– exploring whether signatures of less massive accretions (∼a hundredth of the mass of the Milky Way at the time of the merger) exist in the inner halo of our Galaxy, by deciphering chemo-kinematic spaces and their substructures.

N-body models of the accretion of one or several satellites on a Milky Way-type galaxy, already available in our group, will be also used and analyzed to accompany and strengthen the analysis of the observational data described above, whose final aim is to answer a very simple, but still very debated question in the scientific community : did our Galaxy have a normal or exceptional accretion history ?