2024ApJ...975....2F

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 $\langle {\rm{[Fe/H]}}\rangle =-{2.17}_{-0.05}^{+0.05}$, ${\sigma }_{{\rm{[Fe/H]}}}={0.33}_{-0.07}^{+0.07}$, and ∇_[Fe/H] = -0.23 ± 0.15 dex ${R}_{e}^{-1}$. 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 $\langle {\rm{[Fe/H]}}\rangle =-{1.95}_{-0.04}^{+0.04}$, ${\sigma }_{{\rm{[Fe/H]}}}\,=\,{0.34}_{-0.05}^{+0.05}$, and ∇_[Fe/H]= -0.46 ± 0.10 dex ${R}_{e}^{-1}$, 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 ≲ 10⁶L_⊙) 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.