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 (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 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 〈 [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 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 ≲ 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.