Stars, as key building blocks of galaxies, retain information about the conditions in which they formed and can therefore be used to trace galaxy formation and evolution. Using data from the HALO7D survey, we measure chemical abundances from stellar spectra of main sequence turn-off stars in the Milky Way (MW) stellar halo. From these abundances, in combination with previously-measured velocities, we show that the chemodynamical distributions of stars along four individual lines-of-sight (LOS) are statistically different from one another, in agreement with a growing body of evidence that suggests the the MW stellar halo is not as well-mixed as often assumed.

With the goal of improving our understanding of the MW merger's history by expanding precise chemodynamics measurements to additional LOS in the Galaxy, we develop a technique for measuring precise proper motions (PMs) of stars in sparse fields by combining archival Hubble Space Telescope (HST) images with Gaia data. The resulting PMs are a median of 2.6 times more precise than Gaia alone in sparse HST images of COSMOS, and we recover PMs for the ~25% of sources that are too faint for Gaia to constrain. This technique also enables us to simulate future missions, such as the Roman Space Telescope. With these simulated observations, we design an observation strategy that significantly improves parallax precision at no cost to PM precision.

Chemical tagging experiments are hindered by unexpected, non-stellar signatures in spectra, some of which can originate in the interstellar medium (ISM). To increase the scientific potential of the APOGEE spectrograph, we present a detailed accounting of light in APOGEE stellar spectra using a data-driven model of red clump stars. These near-infrared, H-band spectra are well-described by this model, though their residuals reveal a wealth of information about the intervening gas and dust in the ISM. We characterize the non-stellar light to measure as many as 84 Diffuse Interstellar Bands in the APOGEE wavelength range, ~74 of which were previously unknown, and show that these ISM-based features are likely impacting stellar chemical abundance measurements.