NGC 4449中的77号星团：卫星星系的核心正在转变为球状星团？
Cluster 77 in NGC 4449: The nucleus of a satellite galaxy being transformed into a globular cluster?
摘要：
我们报告了在我们的HST ACS B，V和I图像的NGC 4449的一个球状星团（GC），似乎与两个蓝色恒星的尾巴。星团质量大（M~1.7x10^6 M_sun），高度扁平（ε~0.24）。从色星等图的解析恒星，我们推断活跃的恒星形成的尾巴在过去的~ 2亿年。在r_e内部的平均投影质量密度与总质量的关系图中，星系团位于GC分布的上端，也就是星系核所在的位置。西北方向的尾巴与HI和红外（尘埃/多环芳烃）辐射的集中有关，这些辐射似乎是缠绕在星系周围的更长的流的一部分。这些性质表明，该星系团可能是一个前富含气体的卫星星系的核心，该星系正在经历NGC 4449的潮汐破坏。如果是这样的话，这个星系团与其他被认为是分裂星系的核，如ωCen（银河系）和G1（M31）相比，处于更早的阶段。
英文摘要：
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~1.7x10^6 M_sun) 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 r_e 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 ωCen (Milky Way) and G1 (M31).
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