Spectroscopy of Globular Clusters in M81

We present moderate-resolution spectroscopy of globular clusters (GCs) aroundthe Sa/Sb spiral galaxy M81 (NGC 3031). Sixteen candidate clusters wereobserved with the Low Resolution Imaging Spectrograph on the Keck I telescope.All are confirmed as bona fide GCs, although one of the clusters appears tohave been undergoing a transient event during our observations. In general, theM81 globular cluster system (GCS) is found to be very similar to the Milky Way(MW) and M31 systems, both chemically and kinematically. A kinematic analysisof the velocities of 44 M81 GCS, (the 16 presented here and 28 from previouswork) strongly suggests that the red, metal-rich clusters are rotating in thesame sense as the gas in the disk of M81. The blue, metal-poor clusters havehalo-like kinematics, showing no evidence for rotation. The kinematics ofclusters whose projected galactocentric radii lie between 4 and 8 kpc suggestthat they are rotating much more than those which lie outside these bounds. Wesuggest that these rotating, intermediate-distance clusters are analogous tothe kinematic sub-population in the metal-rich, disk GCs observed in the MW andwe present evidence for the existence of a similar sub-population in themetal-rich clusters of M31. With one exception, all of the M81 clusters in oursample have ages that are consistent with MW and M31 GCs. One cluster may be asyoung as a few Gyrs. The correlations between absorption-line indicesestablished for MW and M31 GCs also hold in the M81 cluster system, at least atthe upper end of the metallicity distribution (which our sample probes). On thewhole, the mean metallicity of the M81 GCS is similar to the metallicity of theMW and M31 GCSs. The projected mass of M81 is similar to the masses of the MWand M31. Its mass profile indicates the presence of a dark matter halo.Comment: 35 pages, including 11 figures and 9 tables. Accepted for publication in the Astronomical Journa