The accretion history of the Milky Way Halo from Massive Spectroscopic Surveys and Cosmological Simulations
The goal of this project is to determine the accretion history of the Milky Way Galaxy through the analysis of massive multi-dimensional data sets from the Gaia satellite and the APOGEE spectroscopic survey. The accretion histories of galaxies are a critical prediction of cosmological models. It impacts galaxy formation in fundamental ways, by shaping their present day structure, kinematics and stellar population content. The Milky Way (MW) is the only galaxy for which observations can be obtained that can be used to infer the detailed accretion history. That is because the MW stellar halo, due to its long dynamical timescale, preserves footprints of the Galaxy’s accretion history. The goal of this project is to explore this precious asset, by combining an analysis of precision data from massive astrometric and spectroscopic surveys of Galactic stellar populations with state-of-the-art cosmological numerical simulations in order to reconstruct the accretion history of the MW. The multi- dimensional observational data upon which the analysis will rely will consist of the following: (i) orbital parameters inferred from positions in 6D phase space based on distances and proper motions from the Gaia satellite and precision radial velocities from SDSS/APOGEE, and (ii) detailed chemical compositions, and ages from the SDSS/APOGEE. The results of the analysis of this unprecedented data set will be contrasted with predictions from the latest high resolution realization of the state-of-the-art EAGLE numerical simulations. Machine-learning techniques will be employed in order to identify substructure in multi-dimensional data associated with different accretion events, distinguishing those from populations formed in situ. By careful consideration of selection function effects from both surveys, it will be possible to infer the fraction of stellar halo stars that were formed in situ vs. those that were accreted. Stellar ages and detailed chemical compositions including abundances of elements sampling all nucleosynthetic pathways will then be used in order to constrain the history of star formation of the in situ halo as well as those of the identified accreted systems. Contrasting the kinematics and chemical compositions of these systems with those of their counterparts in EAGLE will make possible the inference of their stellar masses and accretion time.
The results of the analysis will be an assessment of the number of systems accreted to the MW halo, their contribution to the halo mass budget, their mass distribution, and the timing of their accretion.