UC Berkeley

Detailed studies of our Local Group benchmark our understanding of galaxy formation. Stellar metallicities are key tracers of the baryonic astrophysics shaping galactic properties, but they are challenging to measure in distant and faint galaxies. Narrow-band imaging offers a powerful way to measure stellar metallicities in systems that push our understanding of dwarf galaxy formation to new regimes in luminosity, star formation history, and environment. In this dissertation, I present hundreds of new stellar metallicities in faint, Local Group dwarf galaxies measured via a novel application of Ca H&K imaging on the Hubble Space Telescope. These metallicities measurements encompass: 1) 463 stars in 13 ultra-faint dwarf galaxies (UFDs) around the Milky Way, which effectively doubles the number of stellar metallicities in all known UFDs, b) 374 stellar metallicities in the quenched field dwarf galaxy Tucana (Mv = -8.8, D ~ 1 Mpc), a factor of ~7 increase over literature spectroscopy, and c) 286 stellar metallicities in two M31 dwarfs Andromeda XVI and And XXVIII where there were previously 0 and 13, respectively. These homogeneous metallicity measurements enable a wide range of science: 1) chemical evolution modeling to put novel constraints on the baryon cycle in UFDs, 2) new metallicity benchmarks for cosmological simulations of the faintest galaxies, 3) high-fidelity metallicity gradients that constrain stellar feedback and dark matter core formation models in dwarf galaxies, 4) ~120 Extremely Metal Poor (EMP; [Fe/H] < -3.0) star candidates that will be priority spectroscopic follow-up targets in the era of Extremely Large Telescopes (ELTs). I conclude with a discussion of interpretive and observational studies that follow from this dissertation.