Abstract
Large spectroscopic and astrometric surveys, such as APOGEE and Gaia, provide us with large multi-dimensional data sets that allow us to study the Milky Way and its system of satellite dwarf galaxies in unprecedented detail. Among their many applications, these new tools afford us potent means by which to look for evidence of the accretion of dwarf galaxies, because the latter are well known to evolve in ways that imprint unique chemical and kinematical signatures in the stars that they contribute to the Milky Way when these satellites are accreted into the Milky Way halo. In this dissertation we report the discovery of a significant accreted population of stars exhibiting unique chemistry (particularly in C+N, Mg, Al, and Ni) and kinematics as probed by APOGEE, a system that is now known as the Gaia-Sausage or Gaia-Enceladus. We detail how this accreted system differentiates itself from the in situ population of Milky Way stars at similarly low metallicities that appears to be related to the Milky Way's thick disk. We also show how the chemical abundance profile of this accreted population suggests that it came from a relatively massive dwarf galaxy progenitor, with a size roughly between that of the Small and Large Magellanic Clouds. We also report that the Triangulum-Andromeda Overdensity, a feature in the outskirts of the Milky Way and long debated to be either a tidal stream or a feature of the Galaxy's outer disk, has multi-element chemical abundance patterns consistent with disk origin. TriAnd may have been perturbed out of the disk midplane due to the passage of a dwarf galaxy, possibly Sagittarius. The Sagittarius dwarf galaxy itself, provides a unique laboratory for studying hierarchical mergers, because its accretion is still ongoing and its stellar debris has yet to phase mix throughout the Milky Way halo. We exploit these properties for two separate investigations. First we use the Sagittarius tidal stream to measure the reflex motion of the sun in a wholly unique way from previous methods, by kinematically identifying the Sagittarius trailing arm in Gaia DR2 and comparing its observed proper motion with the most sophisticated models of the Sagittarius stream to constrain the effect of the sun's motion. We also develop a new 6D, positional and kinematical method for tracing the Sagittarius stream according to its motion in its orbital plane. Using APOGEE's precise chemical abundances, we then undertake the most comprehensive survey of abundance gradients along the Sagittarius stream and show how these can be used to reconstruct the metallicity and abundance gradients within the Sagittarius progenitor galaxy. Through these four investigations we demonstrate how modern, large astronomical surveys are opening new windows to the Galaxy's cannibalistic history, making it possible to understand in even greater detail the effects of dwarf galaxy accretion on both their host galaxy and the evolution from satellite to tidally dissolved halo substructure.
