UC San Diego

Galactic Archeologists reconstruct the formation and evolution of the Milky Way by studying its stars. Enabled by large-scale surveys over the past 5 decades (e.g Hipparcos, 2MASS, Pan-STARRS, SDSS, Gaia), they have constructed detailed and precise spatial, kinematic, and chemical maps of the Galaxy using billions of stars. Through these maps, a rich structure has been revealed: a young metal poor disk, an old metal-poor thick disk, an old stellar halo full of debris from disrupted satellite galaxies, a Galactic bar/bulge, and a dark matter halo. These observations offer opportunities for direct comparison with state-of-the-art galaxy formation models. However, the lowest-mass stars and brown dwarfs have been overlooked in these studies. These are low-mass (<0.1 solar masses) objects that are not massive enough to sustain hydrogen fusion, hence they cool down with time. Additionally, they are more than 10,000 times intrinsically fainter compared to the Sun, hence their detection has been limited to the local Solar neighborhood (>100 pc). In this thesis, I present a sample of 164 distant (1-2 kpc) UCDs discovered in the Hubble Space Telescope WFC3 Infrared Spectroscopic Parallel (WISP) Survey and 3D-HST. I model the observed luminosity function using population simulations to place constraints on scaleheights, vertical velocity dispersions, and population ages as a function of spectral type, consistent with prior simulations that predict that L-type dwarfs are on average a younger and less dispersed population. I use this population simulation framework to predict the UCD yield in the JWST PASSAGES survey, a similar and deeper survey to WISPS and 3D-HST, and find that it will produce a comparably-sized UCD sample, albeit dominated by thick disk and halo sources. By adding a set of recently-developed metal-poor models of UCDs to my modeling framework, I predict the expected counts of UCDs in three upcoming surveys such as the Euclid Telescope, the Nancy Grace Roman Observatory, and the Vera Rubin Observatory. I find that these surveys will find millions of UCDs to 10 kpc, allowing us to better probe the Milky Way Structure. In the last chapter of this thesis, I explore the detectability of gaps in globular cluster streams with the Roman telescope as an avenue to constrain the nature of dark matter. I find that Roman will find gaps in Andromeda and other external galaxies up to 3 Mpc, allowing us to directly test galaxy formation models that include dark matter.