The M33 Globular Cluster System with PAndAS Data: The Last Outer Halo Cluster?

TL;DR

This study searches for outer halo star clusters in M33 using PAndAS data, finding only one new cluster among many candidates, and discusses implications for M33's accretion history and interactions with M31.

Contribution

First comprehensive search for outer halo clusters in M33 using wide-field PAndAS data, identifying one new cluster and analyzing its properties.

Findings

Only one new outer halo cluster found in M33.

Most candidate clusters are background contaminants.

M33 has fewer outer halo clusters compared to M31, suggesting different interaction histories.

Abstract

We use CFHT/MegaCam data to search for outer halo star clusters in M33 as part of the Pan-Andromeda Archaeological Survey (PAndAS). This work extends previous studies out to a projected radius of 50 kpc and covers over 40 square degrees. We find only one new unambiguous star cluster in addition to the five previously known in the M33 outer halo (10 kpc <= r <= 50 kpc). Although we identify 2440 cluster candidates of various degrees of confidence from our objective image search procedure, almost all of these are likely background contaminants, mostly faint unresolved galaxies. We measure the luminosity, color and structural parameters of the new cluster in addition to the five previously-known outer halo clusters. At a projected radius of 22 kpc, the new cluster is slightly smaller, fainter and redder than all but one of the other outer halo clusters, and has g' ~ 19.9, (g'-i') ~ 0.6, concentration parameter c ~ 1.0, a core radius r_c ~ 3.5 pc, and a half-light radius r_h ~ 5.5 pc. For M33 to have so few outer halo clusters compared to M31 suggests either tidal stripping of M33's outer halo clusters by M31, or a very different, much calmer accretion history of M33.