We present new high signal - to - noise spectroscopic data on the M31 globular cluster ( GC ) system , obtained with the Hectospec multifiber spectrograph on the 6.5m MMT .
More than 300 clusters have been observed at a resolution of 5Å and with a median S/N of 75 per Å , providing velocities with a median uncertainty of 6 km s ^ { -1 } .
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 .
These match well the metallicities of 24 M31 clusters determined from HST color - magnitude diagrams , the differences having an rms of 0.2 dex .
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 , possibly with minor peaks at [ Fe/H ] = -1.4 , -0.7 and -0.2 , 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 , those between 2 and 6 Gyr .
Moreover , we find that the mean ages of the old GCs are remarkably constant over about a decade in metallicity ( -0.95 \buildrel < \over { \sim } [ Fe/H ] \buildrel < \over { \sim } 0.0 ) .