The Metallicity Distribution Functions of Faint M31 Satellites

Faint M31 satellites are among the few systems that allow us to study low-mass galaxy formation physics in detail outside of the Milky Way (MW) halo. Though HST is systematically measuring the star formation histories (SFHs) and proper motions of all M31 satellites, very few faint M31 satellites have measured resolved star metallicities, due to a paucity of bright, spectroscopically accessible stars. We propose deep WFC3/UVIS F395N (Ca H & K) imaging to measure metallicities (to a precision of ~0.1-0.2 dex) for ~100 individual red giants, a number comparable to typically MW satellites, in 3 faint M31 satellites. This order-of-magnitude improvement enables the first detailed study of metallicity distribution functions (MDFs) in faint M31 satellites. As shown in MW satellites, MDFs uniquely encode the signature of baryonic processes that shape a galaxy's evolution (e.g., inflows, outflows, stripping). We will (1) characterize the MDFs for each system and compare them to each other and to MW satellites; (2) combine our MDFs with HST-based SFHs and orbital histories to constrain the baryonic processes and environmental mechanisms that shaped their evolution; and (3) publish stellar metallicity catalogs for community spectroscopic followup now (brightest stars) or with ELTs.

This program will provide new insight into the formation of faint satellites galaxies outside the MW halo. Measuring high-fidelity MDFs is only possible due to the excellent blue-sensitivity and angular resolution of HST -- no other ground- or space-based facility can acquire this data. This program will demonstrate how HST can provide MDFs for faint galaxies throughout the M31 ecosystem.