2004ApJ...603...87M

We show that there is a subsystem of the M31 globular clusters with thin-disk kinematics. These clusters span the entire metallicity range of the M31 globular cluster system, in contrast to the (thick) disk globular 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 disk system suggests that there was a relatively large thin disk in place very early in M31's history. Accurate measures of the ages of these clusters will constrain the epoch of disk formation in M31. There is currently no strong evidence for differences in age between Milky Way and M31 globular clusters. While age differences are subtle for old populations, it is unlikely that disk clusters with [Fe/H] around -2.0 were formed after significant star formation began in the galaxy, as the protocluster gas would be enriched by supernova ejecta. Thus it is likely that M31 had a rather large disk in place at early epochs. The very existence of such a cold disk means that M31 has suffered no mergers with an object of 10% or more of the disk mass since the clusters were formed. This makes the suggestion of Brown et al. that M31 could have suffered an equal-mass merger 6-8 Gyr ago less viable.