The Outer Halo Globular Clusters of M31

TL;DR

This study analyzes three outer halo globular clusters of M31 using Keck/HIRES spectra, confirming their association, and reveals a declining velocity dispersion, old ages, and metallicity gradients in the system.

Contribution

First spectroscopic analysis of outer halo globular clusters of M31 beyond 80 kpc, providing insights into their kinematics and metallicity distribution.

Findings

Confirmed association of clusters with M31 system.

Detected declining velocity dispersion with radius.

Identified old ages and metal-poor nature of clusters.

Abstract

We present Keck/HIRES spectra of 3 globular clusters in the outer halo of M31, at projected distances beyond ~80 kpc from M31. The measured recession velocities for all 3 globular clusters confirm their association with the globular cluster system of M31. We find evidence for a declining velocity dispersion with radius for the globular cluster system. Their measured internal velocity dispersions, derived virial masses and mass-to-light ratios are consistent with those for the bulk of the M31 globular cluster system. We derive old ages and metallicities which indicate that all 3 belong to the metal-poor halo globular cluster subpopulation. We find indications that the radial gradient of the mean metallicity of the globular cluster system interior to 50 kpc flattens in the outer regions, however it is still more metal-poor than the corresponding field stars at the same (projected) radius.