Exploring the Properties of the M31 Halo Globular Cluster System

TL;DR

This study investigates the properties of M31's outer halo globular clusters up to 120kpc, revealing a flattened density profile and uniform colors beyond 30kpc, supporting the accretion origin of these clusters.

Contribution

It provides new insights into the spatial distribution, metallicity, and structural parameters of M31's outer halo GCs, including the discovery of extended clusters and their implications.

Findings

Flattening of GC density profile at ~30 kpc

Outer halo GCs are uniformly blue with little metallicity gradient

Discovery of 13 extended clusters with properties similar to Milky Way Palomar GCs

Abstract

Following on from our discovery of a significant population of M31 outer halo globular clusters (GCs), and updates to the Revised Bologna Catalogue of M31 GCs, we investigate the GC system of M31 out to an unprecedented radius (~120kpc). We derive various ensemble properties, including the magnitude, colour and metallicity distributions, as well as the GC number density profile. One of our most significant findings is evidence for a flattening in the radial GC number density profile in the outer halo. Intriguingly, this occurs at a galactocentric radius of ~2 degrees (~30 kpc) which is the radius at which the underlying stellar halo surface density has also been shown to flatten. The GCs which lie beyond this radius are remarkably uniform in terms of their blue (V-I)o colours, consistent with them belonging to an ancient population with little to no metallicity gradient. Structural parameters are also derived for a sample of 13 newly-discovered extended clusters (ECs) and we find the lowest luminosity ECs have magnitudes and sizes similar to Palomar-type GCs in the Milky Way halo. We argue that our findings provide strong support for a scenario in which a significant fraction of the outer halo GC population of M31 has been accreted.