The Star Formation Histories of Different Components of M31's Halo from Photometric Estimates of Fe and $α$ Abundances

Abstract

We present the chemical-abundance distributions and star-formation histories of the M31 halo components using photometric Fe and $α$ abundance estimates derived from the PAndAS broad-band $g$ and $i$ filters and the Subaru/HSC $NB515$ narrow-band filter. We selected about 76,400 candidate M31 giant stars and constructed a synthetic photometric training dataset based on model isochrones. We estimated elemental abundances and errors using random forest regression models. We identify various halo components, including about 32,500 stars in the ``smooth" halo (where ``smooth" refers to the complete sample with all the known substructures removed), and infer their star-formation histories from comparisons of their abundance distributions. From the radial abundance gradients, we find that [Fe/H] decreases while [$α$/Fe] increases toward the outer halo. Including substructures further enhances the radial abundance contrast, suggesting that the smooth halo stars are accreted at relatively early times, before the progenitor galaxies produce large numbers of Type Ia supernovae. Both M31 and the Milky Way show similar radial gradient trends, but M31 exhibits higher [Fe/H] and lower [$α$/Fe] values overall, suggesting that the smooth halo of M31 may possess a larger fraction of relatively younger stars associated with more recent merger events. Future photometric observations with additional narrow-band filters and larger spectroscopic samples are expected to provide a better understanding of the formation and evolution history of M31.