Star Clusters in M31: VII. Global Kinematics and Metallicity Subpopulations of the Globular Clusters

TL;DR

This study analyzes the spatial, kinematic, and metallicity properties of M31's globular clusters, revealing three metallicity-based groups with distinct dynamics and spatial distributions, and highlighting similarities with the Milky Way's GC system.

Contribution

It provides a homogeneous spectroscopic analysis that clarifies the chemodynamical subpopulations of M31's globular clusters, correcting previous misconceptions about their rotation.

Findings

Three metallicity groups identified with distinct spatial and kinematic properties.

Metal-rich clusters resemble the disk of M31, while metal-poor clusters show mild rotation.

Similarities between M31 and Milky Way GC systems suggest common formation processes.

Abstract

We carry out a joint spatial-kinematical-metallicity analysis of globular clusters (GCs) around the Andromeda Galaxy (M31), using a homogeneous, high-quality spectroscopic dataset. In particular, we remove the contaminating young clusters that have plagued many previous analyses. We find that the clusters can be divided into three major metallicity groups based on their radial distributions: (1) an inner metal-rich group ([Fe/H] > -0.4), (2) a group with intermediate metallicity (with median [Fe/H]=-1), (3) and a metal-poor group, with [Fe/H] < -1.5. The metal-rich group has kinematics and spatial properties like the disk of M31, while the two more metal-poor groups show mild prograde rotation overall, with larger dispersions - in contrast to previous claims of stronger rotation. The metal-poor GCs are the least concentrated group; such clusters occur five times less frequently in the central bulge than do clusters of higher metallicity. Despite some well-known differences between the M31 and Milky Way GC systems, our revised analysis points to remarkable similarities in their chemodynamical properties, which could help elucidate the different formation stages of galaxies and their GCs. In particular, the M31 results motivate further exploration of a metal-rich GC formation mode in situ, within high-redshift, clumpy galactic disks.