Elemental abundances in M31: Individual and Coadded Spectroscopic Fe H and α Fe throughout the M31 Halo with SPLASH by J. Leigh Wojno et al. on Tuesday 29 November

We present spectroscopic chemical abundances of red giant branch (RGB) stars

in Andromeda (M31), using medium resolution ($R\sim6000$) spectra obtained via

the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo

(SPLASH) survey. In addition to individual chemical abundances, we coadd low

signal-to-noise ratio (S/N) spectra of stars to obtain a high enough to measure

average [Fe/H] and [$\alpha$/Fe] abundances. We obtain individual and coadded

measurements for [Fe/H] and [$\alpha$/Fe] for M31 halo stars, covering a range

of 9--180 kpc in projected radius from the center of M31. With these

measurements, we greatly increase the number of outer halo ($R_{\mathrm{proj}}

> 50$ kpc) M31 stars with spectroscopic [Fe/H] and [$\alpha$/Fe], adding

abundance measurements for 45 individual stars and 33 coadds from a pool of an

additional 174 stars. We measure the spectroscopic metallicity ([Fe/H])

gradient, finding a negative radial gradient of $-0.0050\pm0.0003$ for all

stars in the halo, consistent with gradient measurements obtained using

photometric metallicities. Using the first measurements of [$\alpha$/Fe] for

M31 halo stars covering a large range of projected radii, we find a positive

gradient ($+0.0026\pm0.0004$) in [$\alpha$/Fe] as a function of projected

radius. We also explore the distribution in [Fe/H]--[$\alpha$/Fe] space as a

function of projected radius for both individual and coadded measurements in

the smooth halo, and compare these measurements to those stars potentially

associated with substructure. These spectroscopic abundance distributions

highlight the substantial evidence that M31 has had an appreciably different

formation and merger history compared to our own Galaxy.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.15288v1