Origin of the Red EArly-type GAlaxies in the Nearby Universe – OREAGAN

Early-type galaxies, lenticulars and ellipticals, are the most powerful probes at low redshift of the hierarchical mass assembly of galaxies. Indeed, these galaxies had the most violent history even at moderate redshifts, with the highest frequency of galaxy mergers that shaped their morphology and kinematics. A number of baryonic substructures in early type galaxies are fossil records of their formation history. These range from decoupled central cores to larger-scale structures in the outer regions. Here we propose to study the fossil records of early-type galaxy formation at all scales simultaneously, in order to optimise the discriminating criteria between the various possible formation mechanisms: major or minor mergers, dry or wet mergers, binary and multiple mergers, and the supply of cold gas accretion from the cosmic web. It will be the first time that all scales of substructures are taken into account in a statistical way: this will provide tight constraints on the formation history of early-type galaxies, which we will compare with predictions from the standard cosmological paradigm. For this purpose, we will first refine the existing and develop new quantitative tracers of the signatures of the mass assembly processes in the outer regions of early-type galaxies, which will be statistically compared to other fossil records in the central regions. We will also implement new schemes for the integration of a multi-phase ISM within state-of-the-art numerical simulations of both generic mergers and in a cosmological context in order to derive a robust description of the faint outer structures and small inner cores simultaneously. These two aspects both critically require a significant support from the ANR, in the form of 2 Post-Doctorates (one in Lyon, one in Saclay) as well as some associated missions and small materials. This will then allow us to gather the resulting products into a single fully self-consistent approach for the study of the interaction history of early-type galaxies, bringing new strong constraints on the scenarios of galaxy formation and evolution.