Detailed Elemental Abundances in the M31 Stellar Halo: Low-Resolution Resolved Stellar Spectroscopy

While measurements of [α/Fe] have been made in the stellar halo of the Milky Way, little is known about detailed chemical abundances in the stellar halo of M31. To make progress with existing telescopes, we apply spectral synthesis to low-resolution DEIMOS spectroscopy (R ∼ 2500 at 7000 \AA) across a wide spectral range (4500 \AA < λ < 9100 \AA). By applying our technique to low-resolution spectra of 135 red giant branch (RGB) stars in 4 MW globular clusters, we demonstrate that our technique reproduces previous measurements from higher resolution spectroscopy. Based on the intrinsic dispersion in [Fe/H] and [α/Fe] of individual stars in our combined cluster sample, we estimate systematic uncertainties of ∼0.11 dex and ∼0.04 dex in [Fe/H] and [α/Fe], respectively. We apply our method to deep, low-resolution spectra of 14 RGB stars in the smooth halo of M31, resulting in higher signal-to-noise per spectral resolution element compared to DEIMOS medium-resolution spectroscopy, given the same exposure time and conditions. We find ⟨[α/Fe]⟩ = 0.49 ± 0.31 dex and ⟨[Fe/H]⟩ = −1.53 ± 0.52 dex for our sample. This implies that the smooth halo field is likely composed of disrupted dwarf galaxies with truncated star formation histories that were accreted early in the formation history of M31.