Spectroscopy of Globular Clusters in M81

We present moderate-resolution spectroscopy of globular clusters around the Sa/Sb spiral galaxy M81 (NGC 3031). Sixteen candidate clusters were observed with the Low Resolution Imaging Spectrograph on the Keck I telescope. All are confirmed as bona fide globular clusters, although one of the clusters appears to have been undergoing a transient event during our observations. In general, the M81 globular cluster system is found to be very similar to the Milky Way and M31 systems, both chemically and kinematically. A kinematic analysis of the velocities of 44 M81 globular clusters (the 16 presented here and 28 from previous work) strongly suggests that the red metal-rich clusters are rotating in the same sense as the gas in the disk of M81. The blue metal-poor clusters have halo-like kinematics, showing no evidence for rotation. The kinematics of clusters whose projected galactocentric radii lie between 4 and 8 kpc suggests that they are rotating much more than those with projected galactocentric radii outside these bounds. We suggest that these rotating, intermediate-distance clusters are analogous to the kinematic subpopulation in the metal-rich, disk globular clusters observed in the Milky Way, and we present evidence for the existence of a similar subpopulation in the metal-rich clusters of M31. With one exception, all of the M81 clusters in our sample have ages that are consistent with Milky Way and M31 globular clusters. One cluster may be as young as a few Gyr. The correlations between absorption-line indices established for Milky Way and M31 globular clusters also hold in the M81 cluster system, at least at the upper end of the metallicity distribution (which our sample probes). On the whole, the mean metallicity of the M81 globular cluster system is similar to the metallicity of the Milky Way and M31 globular cluster systems. To within a factor of 2, the projected mass of M81 is similar to the masses of the Milky Way and M31. Its mass profile indicates the presence of a dark matter halo.