Global Properties of M31's Stellar Halo from the SPLASH Survey: II. Metallicity Profile

TL;DR

This study maps the metallicity distribution of M31's stellar halo, revealing a continuous gradient and insights into its formation history through analysis of spectroscopically confirmed RGB stars across various distances.

Contribution

It provides the first detailed metallicity profile of M31's halo from 9 to 175 kpc, showing a persistent gradient and evidence of multiple progenitors in its assembly history.

Findings

Metallicity decreases with increasing radius in M31's halo.

Tidal debris removal does not significantly change the metallicity gradient.

Outer halo likely formed from multiple smaller progenitors.

Abstract

We present the metallicity distribution of red giant branch (RGB) stars in M31's stellar halo, derived from photometric metallicity estimates for over 1500 spectroscopically confirmed RGB halo stars. The stellar sample comes from 38 halo fields observed with the Keck/DEIMOS spectrograph, ranging from 9 to 175 kpc in projected distance from M31's center, and includes 52 confirmed M31 halo stars beyond 100 kpc. While a wide range of metallicities is seen throughout the halo, the metal-rich peak of the metallicity distribution function becomes significantly less prominent with increasing radius. The metallicity profile of M31's stellar halo shows a continuous gradient from 9 to ~100 kpc, with a magnitude of -0.01 dex/kpc. The stellar velocity distributions in each field are used to identify stars that are likely associated with tidal debris features. The removal of tidal debris features does not significantly alter the metallicity gradient in M31's halo: a gradient is maintained in fields spanning 10 to 90 kpc. We analyze the halo metallicity profile, as well as the relative metallicities of stars associated with tidal debris features and the underlying halo population, in the context of current simulations of stellar halo formation. We argue that the large scale gradient in M31's halo implies M31 accreted at least one relatively massive progenitor in the past, while the field to field variation seen in the metallicity profile indicates that multiple smaller progenitors are likely to have contributed substantially to M31's outer halo.