Cluster 77 in NGC 4449: The nucleus of a satellite galaxy being transformed into a globular cluster?
Abstract
We report the discovery in our HST ACS 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~1.7x106 Msun) and highly flattened (ε~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 re versus total mass the cluster lies at the upper end of the GC distribution, where galaxy nuclei are. The north-west tail is associated with a concentration of HI and infrared (dust/PAHs) 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).