Compact star clusters in M81. II. Two populations

We present the color and magnitude distributions of compact star clusters d iscovered in the early-type spiral galaxy M81, from Hubble Space Telescope Wide Field Planetary Camera 2 B, V, and I-band imaging. These clearly show the presence of two cluster populations-a typical red globular cluster syst em, as well as a young population, which was not previously known to exist in M81. While the young clusters in M81 are not as massive as those found i n merging and starburst galaxies (for example, its neighbor M82), they more closely resemble the populous clusters in the Magellanic Clouds and M33 an d similarly are found in all environments of M81. Further, the young cluste rs are more compact than their counterparts in M82, with core radii between 0.4 and 4.3 pc. The color-color diagram indicates that young cluster forma tion may have "turned on" similar to 600 Myr ago and continued until 6 Myr ago. The timescale of continued cluster formation in M81 coincides both wit h independent dynamical estimates for the last tidal encounter between M81 and M82, as well as with a peak in cluster formation recently discovered in the fossil starburst region B in M82. This suggests that interactions may have sparked the recent cluster formation in M81.