Elemental Abundances in M31: A Comparative Analysis of Iron and Alpha Element Abundances in the Outer Disk, Giant Stellar Stream, and Inner Halo of M31

TL;DR

This study analyzes elemental abundances in M31's outer disk, stellar stream, and inner halo, revealing differences in metallicity and alpha enhancement that inform galaxy formation and merger history.

Contribution

It provides the first detailed comparison of [Fe/H] and [$ ext{alpha}$/Fe] across multiple M31 regions using spectral synthesis of low-resolution stellar spectra.

Findings

Outer regions are alpha-enhanced with high [$ ext{alpha}$/Fe] values.

Halo fields are more metal-poor than the stream and disk.

Evidence suggests different formation mechanisms for inner and outer halo.

Abstract

We measured [Fe/H] and [$\alpha$/Fe] using spectral synthesis of low-resolution stellar spectroscopy for 70 individual red giant branch stars across four fields spanning the outer disk, Giant Stellar Stream (GSS), and inner halo of M31. Fields at M31-centric projected distances of 23 kpc in the halo, 12 kpc in the halo, 22 kpc in the GSS, and 26 kpc in the outer disk are $\alpha$-enhanced, with $\langle$[$\alpha$/Fe]$\rangle$ = 0.43, 0.50, 0.41, and 0.58, respectively. The 23 kpc and 12 kpc halo fields are relatively metal-poor, with $\langle$[Fe/H]$\rangle$ = $-$1.54 and $-$1.30, whereas the 22 kpc GSS and 26 kpc outer disk fields are relatively metal-rich with $\langle$[Fe/H]$\rangle$ = $-$0.84 and $-$0.92, respectively. For fields with substructure, we separated the stellar populations into kinematically hot stellar halo components and kinematically cold components. We did not find any evidence of an [$\alpha$/Fe] gradient along the high surface brightness core of the GSS between $\sim$17$-$22 kpc. However, we found tentative suggestions of a negative [$\alpha$/Fe] gradient in the stellar halo, which may indicate that different progenitor(s) or formation mechanisms contributed to the build up of the inner versus outer halo. Additionally, the [$\alpha$/Fe] distribution of the metal-rich ([Fe/H] $>$ $-$1.5), smooth inner stellar halo (r$_{\rm{proj}}$ $\lesssim$ 26 kpc) is inconsistent with having formed from the disruption of progenitor(s) similar to present-day M31 satellite galaxies. The 26 kpc outer disk is most likely associated with the extended disk of M31, where its high $\alpha$-enhancement provides support for an episode of rapid star formation in M31's disk, possibly induced by a major merger.