M31’s Undisturbed Thin Disk of Globular Clusters

We show that there is a subsystem of the M31 globular clusters with THIN DISKkinematics. These clusters span the entire metallicity range of the M31globular cluster system, in contrast to the (thick) disk globulars in the MilkyWay which are predominantly metal-rich. Disk globular clusters are found acrossthe entire disk of M31 and form around 40% of the clusters projected on itsdisk. The existence of such a disk system suggests that there was a relativelylarge thin disk in place very early in M31's history. Accurate measures of theages of these clusters will constrain the epoch of disk formation in M31. Thereis currently no strong evidence for differences in age between Milky Way andM31 globulars. While age differences are subtle for old populations, it isunlikely that disk clusters with [Fe/H] around -2.0 were formed aftersignificant star formation began in the galaxy, as the proto-cluster gas wouldbe enriched by supernova ejecta. Thus it is likely that M31 had a rather largedisk in place at early epochs. The very existence of such a cold disk means that M31 has suffered no mergerswith an object of 10% or more of the disk mass since the clusters were formed.This makes Brown et al (2003)'s suggestion that M31 could have suffered anequal-mass merger 6-8 Gyr ago less viable.Comment: 60 pages including 17 figures; submitted to Ap