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

Abstract

We present ∼300 stellar metallicity measurements in two faint M31 dwarf galaxies, Andromeda XVI (M _V = ‑7.5) and Andromeda XXVIII (M _V = –8.8), derived using metallicity-sensitive calcium H and K narrowband 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 Milky Way ultrafaint dwarf galaxies 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]=0.34‑0.05+0.05 , 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 (MDF) 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.