Elemental Abundances in M31: Properties of the Inner Stellar Halo*

Ivanna Escala,Karoline M Gilbert,Emily C Cunningham,Evan N Kirby,Puragra Guhathakurta,Jennifer Wojno

doi:10.3847/1538-4357/abb474

Abstract

Abstract We present measurements of [Fe/H] and [α/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 ( 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 [α/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 and . Between 8 and 34 kpc, the inner halo has a steep [Fe/H] gradient (−0.025 ± 0.002 dex kpc−1) and negligible [α/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.

Highlights

In the ΛCDM cosmological paradigm, L galaxies like the Milky Way (MW) and Andromeda (M31) form through hierarchical assembly (e.g., White & Rees 1978)
We show the locations of pencilbeam Hubble Space Telescope (HST)/ACS fields (Brown et al 2009), some of which overlap with DEIMOS fields (Escala et al 2020), from which Brown et al (2006, 2007, 2008) derived stellar age distributions
This resulted in the identification of two kinematically and chemically distinct MW stellar halo populations characterized by low-[Mg/Fe] with negligible Galactic rotation (LMg) and high-[Mg/Fe] with significant Galactic rotation (HMg)

Summary

Introduction

In the ΛCDM cosmological paradigm, L galaxies like the Milky Way (MW) and Andromeda (M31) form through hierarchical assembly (e.g., White & Rees 1978). Abundance (O, Ne, Mg, Si, S, Ar, Ca, and Ti) and iron (Fe) abundance encode information concerning the relative timescales of Type Ia and core-collapse supernovae (e.g., Gilmore & Wyse 1998), such that galactic systems with different evolutionary histories will have distinct chemical abundance patterns. In this way, stellar halos serve as fossil records of a galaxy’s accretion history. This theory has been extensively put into practice in the MW, where the differing patterns of [α/Fe] and [Fe/H] between its stellar halo and satellite dwarf galaxies have revealed their fundamentally incompatible enrichment histories (Shetrone et al 2001; Venn et al 2004)

Results

Discussion

Conclusion