Stellar Metallicities and Gradients in the Faint M31 Satellites Andromeda XVI and Andromeda XXVIII

We present ~300 stellar metallicity measurements in two faint M31 dwarf galaxies, Andromeda XVI (MV = -7.5) and Andromeda XXVIII (MV = -8.8) derived using metallicity-sensitive Calcium H & K narrow-band Hubble Space Telescope imaging. These are the first individual stellar metallicities in And~XVI (95 stars). Our And~XXVIII sample (191 stars) is a factor of ~15 increase over literature metallicities. For And~XVI, we measure [Fe/H] = -2.17+0.05-0.05, σ[Fe/H]=0.33+0.07-0.07, and ∇[Fe/H] = -0.23±0.15 dex Rₑ⁻¹. We find that And XVI is more metal-rich than MW UFDs of similar luminosity, which may be a result of its unusually extended star formation history. For And XXVIII, we measure [Fe/H] = -1.95+0.04-0.04, σ[Fe/H]=0.34+0.07-0.07, and ∇[Fe/H] = -0.46 ± 0.10~dex~Rₑ⁻¹, placing it on the dwarf galaxy mass-metallicity relation. Neither galaxy has a metallicity distribution function with an abrupt metal-rich truncation, suggesting that star formation fell off gradually. The stellar metallicity gradient measurements are among the first for faint (L 10⁶~L_) galaxies outside the Milky Way halo. Both galaxies' gradients are consistent with predictions from the FIRE simulations, where an age-gradient strength relationship is the observational consequence of stellar feedback that produces dark matter cores. We include a catalog for community spectroscopic follow-up, including 19 extremely metal poor ([Fe/H] < -3.0) star candidates, which make up 7% of And~XVI's MDF and 6% of And~XXVIII's.