The M31 globular cluster system

The M31 globular cluster system is at least a factor of two more populous than that of the Milky Way. Recent studies find that the distribution in metallicity is bimodal, with the majority of the clusters belonging to a metal poor population with [Fe/H]=-1.5 dex, and 1/3 of the population belonging to a metal rich group at -0.5 dex. The metal rich clusters are markedly concentrated relative to the metal poor clusters, but the entire cluster system, and it subpopulations, shows clear rotation. Claims that the metal rich cluster population are a few Gyr younger than their metal poor counterparts remain, though no intermediate age stars are seen in HST color-magnitude diagrams. We also consider the luminous globular cluster G1 as well as possible explanations for its large velocity dispersion and core rotation. Some differences relative to the Milky Way remain: enhanced nitrogen abundance, and the large population of luminous X-ray sources, but after decades of study, the M31 globular cluster may still be considered to be similar to that of the Galaxy.