Elemental Abundances in M31: Properties of the Inner Stellar Halo

TL;DR

This study measures elemental abundances in 128 stars in M31's stellar halo, revealing a steep metallicity gradient and chemical differences from current dwarf satellites, offering insights into the galaxy's formation history.

Contribution

It provides the largest sample of [Fe/H] and [$eta$/Fe] measurements in M31's halo, combining new and existing data to analyze its chemical properties and formation scenarios.

Findings

Inner halo has a steep [Fe/H] gradient of -0.025 dex/kpc.

Chemical abundances largely differ from present-day dwarf spheroidals.

22 stars show dSph-like chemical patterns, suggesting accretion from similar or larger galaxies.

Abstract

We present measurements of [Fe/H] and [$\alpha$/Fe] for 128 individual red giant branch stars (RGB) in the stellar halo of M31, including its Giant Stellar Stream (GSS), obtained using spectral synthesis of low- and medium-resolution Keck/DEIMOS spectroscopy ($R \sim 3000$ and 6000, respectively). We observed four fields in M31's stellar halo (at projected radii of 9, 18, 23, and 31 kpc), as well as two fields in the GSS (at 33 kpc). In combination with existing literature measurements, we have increased the sample size of [Fe/H] and [$\alpha$/Fe] measurements from 101 to a total of 229 individual M31 RGB stars. From this sample, we investigate the chemical abundance properties of M31's inner halo, finding $\langle$[Fe/H]$\rangle$ = $-$1.08 $\pm$ 0.04 and $\langle$[$\alpha$/Fe]$\rangle$ = 0.40 $\pm$ 0.03. Between 8--34 kpc, the inner halo has a steep [Fe/H] gradient ($-$0.025 $\pm$ 0.002 dex kpc$^{-1}$) and negligible [$\alpha$/Fe] gradient, where substructure in the inner halo is systematically more metal-rich than the smooth component of the halo at a given projected distance. Although the chemical abundances of the inner stellar halo are largely inconsistent with that of present-day dwarf spheroidal (dSph) satellite galaxies of M31, we identified 22 RGB stars kinematically associated with the smooth component of the stellar halo that have chemical abundance patterns similar to M31 dSphs. We discuss formation scenarios for M31's halo, concluding that these dSph-like stars may have been accreted from galaxies of similar stellar mass and star formation history, or of higher stellar mass and similar star formation efficiency.