title: Unveiling the mass assembly history of the Milky Way from its stellar halo
Creators: name: Horta Darrington, D
Publisher: name: Liverpool John Moores University
Year: 2022

description: Stellar halos of galaxies retain crucial clues to their mass assembly history. It is in these galactic components that the remains of cannibalised galactic building blocks are deposited. For the case of the Milky Way, the opportunity to analyse the stellar halo’s structure on a star-by-star basis in a multi-faceted approach provides a basis from which to infer its past and assembly history in unrivalled detail. Moreover, the insights that can be gained about the formation of the Galaxy not only help constrain the evolution of our Milky Way, but may also help place constraints on the formation of other disc galaxies in the Universe. This thesis aims to make progress toward answering the most fundamental question in the field of Galactic archaeology: “How did the Milky Way form?” Through the effort to answer this question, this thesis presents new insights into aspects of the history of assembly and evolution of our Galaxy and measurements of the structure of various of its Galactic components. Providing further insight into the accretion history and mass assembly of the Milky Way, I present a detailed analysis of the properties of Milky Way halo stars in the heart of the Galaxy contained in both the APOGEE and Gaia data sets. I present evidence for the discovery of a new halo substructure (whose progenitor we attribute the name of ”Heracles”) that, given its chemical composition and dynamical properties, is likely to be the debris from a major building block of the Milky Way. I also compare its properties with expectations from the EAGLE numerical simulations to ascertain its nature, and make a quantitative prediction of the stellar mass of the disrupted satellite galaxy to comprise approximately one third of the estimated total stellar halo mass. To ascertain the reality and nature of halo substructures, and to place further constraints on the mass assembly history of the Galaxy, I perform a detailed qualitative and quantitative analysis of the chemical compositions of halo substructures in the Milky Way with APOGEE and Gaia data. The findings from this study revealed that many halo substructures identified in recent years, conjectured to be the debris from individual satellite accretions, likely belong to the Gaia-Enceladus/Sausage accretion event. They also showed that the Heracles halo substructure is statistically different from in situ populations given its chemical compositions, further confirming its accreted nature. To understand how much mass dissolved and/or evaporated globular clusters (GC) contribute to the total stellar halo mass budget, I perform a density modelling analysis of stellar halo populations. By identifying GC escapees using a Gaussian mixture modelling and chemical tagging procedure, I model their density distribution accounting for the APOGEE selection function and assess their ratio to the halo field. The main finding of this work showed that in the inner ∼2-3 kpc from the Galactic centre, there is a much higher incidence of dissolved/evaporated GC stars that is on the order of five to six times larger than in the outer ∼10 kpc region. In order to decipher the origin of the Galactic GC system, I undertook a study aimed at comparing the chemical composition of previously categorised GC subgroups classified based on their orbits. More specifically, by determining a homogeneous sample of GC star members in the APOGEE DR16 survey, and comparing the mean [α/Fe] and [Fe/H] abundances of GCs with field populations, I was able to place constraints on the origin of GCs in the Milky Way, that in turn help place constraints on the accretion history of the Galaxy. The above results place constraints on our current understanding of the accretion and mass assembly history of the Milky Way. In discovering new halo substructures, assessing the reality of known ones, modelling the density of stars contributed from dissolved/evaporated GCs, and deciphering the origin of the Galactic GC system, I have tackled the question of “How did the Milky Way form?” from numerous different angles. All the findings contained in this thesis help pave the way for future work towards the goal of fully reconstructing the assembly history of our Galaxy and using that understanding to formulate robust and general models for the formation of disc galaxies.