The Milky Way -- An Immigrant Story, or Unraveling the Galactic Halo with the H3 Survey

Abstract

A hallmark prediction of the standard cosmological model (ΛCDM) is that galaxies like our Milky Way grow by assimilating smaller, immigrant galaxies throughout their history. The outer reaches of our Galaxy -- the stellar halo -- are predicted to be the melting pot for stars that were born elsewhere, but now call the Milky Way their home. Despite being scattered across the Galaxy, immigrant stars retain memory of their common origin that may be accessed via their shared chemistry and dynamics. The long-held aspiration of tracing every star in the halo to a distinct accreted galaxy has only recently become possible thanks to the European Space Agency's ongoing Gaia astrometric mission. This thesis presents results from the H3 Spectroscopic Survey -- the first dedicated halo survey that leverages Gaia astrometry to efficiently target stars in the distant Galaxy. Combined with Gaia, H3 is measuring the dynamics and chemistry required to reconstruct the Galaxy's history. Here we present a comprehensive inventory of our Galaxy out to 50 kpc, mapping debris from various known as well as multiple previously undiscovered dwarf galaxies. We demonstrate that the halo is almost entirely comprised of accreted dwarfs, and stars heated out of the disk during mergers with these galaxies. The mass budget is dominated by a handful of massive systems, confirming a long-standing ΛCDM prediction and explaining the relatively high metallicity of the halo we derive compared to previous studies. With this new view of the assembly history of the Galaxy, we can now begin building realistic models of the Milky Way. We present the first tailored N-body simulations of the Milky Way's last major merger, which transpired 10 billion years ago with the Gaia-Sausage Enceladus (GSE) galaxy. These simulations self-consistently explain a variety of phenomena across the Galaxy (e.g., the triaxial shape of the inner halo), and show GSE infused the Galaxy with ~20% of its present-day dark matter content. The disrupted halo dwarfs provide convenient access to high-redshift" stellar chemistry. The star-formation in these dwarfs was abruptly truncated when they were destroyed by the Galaxy, and so their chemical abundances reflect the conditions of the early Universe. We exploit this fact to present the first stellar metallicity gradient in a z~2 star-forming galaxy (GSE), finding it to be flat, and validating inside-out galaxy formation models. We measure heavy element abundances in the GSE and Kraken dwarf galaxies to constrain the unknown production sites of these elements. Candidate sites have distinct timescales – e.g., core-collapse supernovae (CCSNe) produce prompt explosions, while neutron star mergers (NSMs) follow a delay time distribution. They may therefore be disentangled using galaxies with comparable mass but differing disruption epochs. We show that both NSMs and CCSNe are required to explain the abundance trends in GSE and Kraken. Finally, we present a comparative population study of the disrupted dwarfs and the Milky Way's surviving satellites. At fixed mass, we show the disrupted dwarfs are more metal-poor and α-enhanced than their surviving peers. We argue that this is because they assembled their mass rapidly, and at higher redshifts. We use these trends to place archaeological constraints on stellar abundances at z~1-3 in a mass regime that lies beyond the reach of even the James Webb Space Telescope, underscoring the immense promise of our newly discovered disrupted dwarfs for such high-z" studies.