Star Clusters in M31: II. Old Cluster Metallicities and Ages from
  Hectospec Data
  We present new high signal-to-noise spectroscopic data on the M31 globular
cluster system, obtained with the Hectospec multifiber spectrograph on the 6.5m
MMT. More than 300 clusters have been observed at a resolution of 5A. The
primary focus of this paper is the determination of mean cluster metallicities,
ages and reddenings. Metallicities were estimated using a calibration of Lick
indices with [Fe/H] provided by Galactic GCs. The metallicity distribution is
not generally bimodal, in strong distinction with the bimodal Galactic globular
distribution. Rather, the M31 distribution shows a broad peak, centered at
[Fe/H]=-1, suggesting that the cluster systems of M31 and the Milky Way had
different formation histories. Ages for clusters with [Fe/H] > -1 were
determined using the automatic stellar population analysis program EZ_Ages. We
find no evidence for massive clusters in M31 with intermediate ages. Moreover,
we find that the mean ages of the old GCs are remarkably constant over about a
decade in metallicity (-0.95 < [Fe/H] < 0.0). (abridged)
