Elemental Abundances in M31: Iron and Alpha Element Abundances in M31's Outer Halo

TL;DR

This study measures iron and alpha element abundances in stars of M31's outer halo, revealing similarities with Milky Way halo stars and confirming a large-scale metallicity gradient extending to 100 kpc.

Contribution

It provides the first spectral synthesis-based abundance measurements for M31's outer halo stars, comparing them with inner halo and dwarf galaxy stars to understand chemical evolution.

Findings

Outer halo stars have alpha abundances similar to M31's dwarf satellites.

Spectral synthesis measurements align with previous photometric metallicity gradients.

Outer halo metallicity decreases with distance, confirming a large-scale gradient.

Abstract

We present [Fe/H] and [$\alpha$/Fe] abundances, derived using spectral synthesis techniques, for stars in M31's outer stellar halo. The 21 [Fe/H] measurements and 7 [$\alpha$/Fe] measurements are drawn from fields ranging from 43 to 165 kpc in projected distance from M31. We combine our measurements with existing literature measurements, and compare the resulting sample of 23 stars with [Fe/H] and 9 stars with [$\alpha$/Fe] measurements in M31's outer halo with [$\alpha$/Fe] and [Fe/H] measurements, also derived from spectral synthesis, in M31's inner stellar halo ($r < $26 kpc) and dSph galaxies. The stars in M31's outer halo have [$\alpha$/Fe] patterns that are consistent with the largest of M31's dSph satellites (And I and And VII). These abundances provide tentative evidence that the [$\alpha$/Fe] abundances of stars in M31's outer halo are more similar to the abundances of Milky Way halo stars than to the abundances of stars in M31's inner halo. We also compare the spectral synthesis-based [Fe/H] measurements of stars in M31's halo with previous photometric [Fe/H] estimates, as a function of projected distance from M31. The spectral synthesis-based [Fe/H] measurements are consistent with a large-scale metallicity gradient previously observed in M31's stellar halo to projected distances as large as 100 kpc.