The structure of star clusters in the outer halo of M31

TL;DR

This study analyzes the structure of star clusters in M31's outer halo using HST imaging, revealing differences between classical globulars and extended clusters, and highlighting the presence of luminous globulars at large galactocentric distances.

Contribution

It provides detailed structural profiles of halo star clusters in M31, including extended and classical globulars, and compares their properties with those in the Milky Way.

Findings

Extended clusters fit King and Sersic profiles with large radii.

60% of classical globulars have cuspy cores with Sersic index 2-6.

Luminous globulars are found out to 100 kpc in M31, unlike in the Milky Way.

Abstract

We present a structural analysis of halo star clusters in M31 based on deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging. The clusters in our sample span a range in galactocentric projected distance from 13 to 100 kpc and thus reside in rather remote environments. Ten of the clusters are classical globulars, while four are from the Huxor et al. (2005, 2008) population of extended, old clusters. For most clusters, contamination by M31 halo stars is slight, and so the profiles can be mapped reliably to large radial distances from their centres. We find that the extended clusters are well fit by analytic King (1962) profiles with ~20 parsec core radii and ~100 parsec photometric tidal radii, or by Sersic profiles of index ~1 (i.e. approximately exponential). Most of the classical globulars also have large photometric tidal radii in the range 50-100 parsec, however the King profile is a less good fit in some cases, particularly at small radii. We find 60% of the classical globular clusters exhibit cuspy cores which are reasonably well described by Sersic profiles of index ~2-6. Our analysis also reinforces the finding that luminous classical globulars, with half-light radii <10 parsec, are present out to radii of at least 100 kpc in M31, which is in contrast to the situation in the Milky Way where such clusters (other than the unusual object NGC 2419) are absent beyond 40 kpc.