Cluster 77 in NGC 4449: The Nucleus of a Satellite Galaxy Being Transformed Into a Globular Cluster?

We report the discovery in our Hubble Space Telescope Advanced Camera for Surveys B , V , and I images of NGC 4449 of a globular cluster (GC) which appears associated with two tails of blue stars. The cluster is massive ( M ∼ 1.7 × 10 6 M ☉ ) and highly flattened ( ∼ 0.24). From the color–magnitude diagrams of the resolved stars we infer active star formation in the tails over the past ∼200 Myr. In a diagram of mean projected mass density inside r e versus total mass the cluster lies at the upper end of the GC distribution, where galaxy nuclei are. The northwest tail is associated with a concentration of H i and infrared (dust/polycyclic aromatic hydrocarbons) emission which appears as part of a much longer stream wrapping around the galaxy. These properties suggest that the cluster may be the nucleus of a former gas-rich satellite galaxy undergoing tidal disruption by NGC 4449. If so, the cluster is seen in an earlier phase compared to other suggested nuclei of disrupted galaxies such as ω Cen (Milky Way) and G1 (M31).