Metallicities of Outer Halo M31 Globular Clusters from Integrated Light Calcium-II Triplet Spectroscopy

TL;DR

This study measures metallicities of outer halo M31 globular clusters using integrated light calcium-II triplet spectroscopy, revealing their generally metal-poor nature and insights into galaxy substructure and accretion history.

Contribution

It introduces a method to determine [Fe/H] ratios for M31 GCs via IL spectroscopy of the CaT lines, expanding metallicity data for outer halo clusters.

Findings

Outer halo GCs are more metal-poor than inner halo GCs.

Some GCs show higher metallicity, indicating possible accretion from dwarf satellites.

Metallicity data constrains the substructure and accretion history of M31's outer halo.

Abstract

This paper presents [Fe/H] ratios for GCs in the outer halo of the Andromeda Galaxy, M31, based on moderate-resolution, integrated light (IL) spectroscopy of the calcium-II triplet (CaT) lines. The CaT strengths are measured by fitting Voigt profiles to the lines and integrating those profiles; integrations of defined bandpasses are also considered. The [Fe/H] ratios are determined using an empirical calibration with CaT line strength, as derived from another sample of M31 GCs that were previously studied at high-resolution. The [Fe/H] ratios for the new GCs reveal that the outer halo GCs are indeed generally more metal-poor than typical inner halo GCs, though there are several more metal-rich GCs that look to have been accreted from dwarf satellites. The metallicities of these GCs also place important constraints on the nature of the substructure in the outer halo and the dwarf satellites that created this substructure.