Star Cluster Candidates in M81

Abstract

We present a catalog of extended objects in the vicinity of M81 based on a set of 24 Hubble Space Telescope Advanced Camera for Surveys Wide Field Camera F814W (I-band) images. We have found 233 good globular cluster (GC) candidates; 92 candidate H II regions, OB associations, or diffuse open clusters; 489 probable background galaxies; and 1719 unclassified objects. We have color data from ground-based g- and r-band MMT Megacam images for 79 galaxies, 125 GC candidates, 7 H II regions, and 184 unclassified objects. The color-color diagram of GC candidates shows that most fall into the range 0.25 < g - r < 1.25 and 0.5 < r - I < 1.25, similar to the color range of Milky Way GCs. Unclassified objects are often blue, suggesting that many of them are likely to be H II regions and open clusters, although a few galaxies and GCs may be among them.