M31’s Undisturbed Thin Disk of Globular Clusters

Abstract

We show that there is a subsystem of the M31 globular clusters withthin-disk kinematics. These clusters span the entire metallicity rangeof the M31 globular cluster system, in contrast to the (thick) diskglobular clusters in the Milky Way, which are predominantly metal-rich.Disk globular clusters are found across the entire disk of M31 and form~40% of the clusters projected on its disk. The existence of such a disksystem suggests that there was a relatively large thin disk in placevery early in M31's history. Accurate measures of the ages of theseclusters will constrain the epoch of disk formation in M31. There iscurrently no strong evidence for differences in age between Milky Wayand M31 globular clusters. While age differences are subtle for oldpopulations, it is unlikely that disk clusters with [Fe/H] around -2.0were formed after significant star formation began in the galaxy, as theprotocluster gas would be enriched by supernova ejecta. Thus it islikely that M31 had a rather large disk in place at early epochs. Thevery existence of such a cold disk means that M31 has suffered nomergers with an object of 10% or more of the disk mass since theclusters were formed. This makes the suggestion of Brown et al. that M31could have suffered an equal-mass merger 6-8 Gyr ago less viable.