超高速恒星的另一种起源
An Alternative Origin for Hypervelocity Stars
摘要：
具有异常高径向速度的晕星（“超高速”星，或HVS）被认为是不受银河系束缚的恒星，起源于恒星系统与银河系中心的超大质量黑洞的引力相互作用。我们研究了最新的HVS编译，并发现在这个黑洞喷射场景中意想不到的特殊性。例如，很大一部分的HVS聚集在狮子座周围，并共享$\sim 100$-2亿年的共同旅行时间。此外，如果像最近的星系形成模型所建议的那样，银河系嵌入在一个$2.5\times 10^{12} h^{-1} M_{\odot}$暗晕中，维里速度为$\sim 220$ km/s，那么它们的速度就不是真正的极端。在这种情况下，在$\sim 50$ kpc处的逃逸速度将是$\sim 600$ km/s，并且很少HVS将是真正未结合的。我们使用数值模拟表明，破坏矮星系可能会贡献晕星的速度高达，有时超过标称逃逸速度的系统。这些恒星排列在一个薄准直的外向“潮汐尾”中，在最近的近心点通过期间从矮星上剥离下来。因此，我们推测一些HVS可能是最近在银河系中心附近被破坏的矮星的潮汐碎片。在这种解释中，HVS的角度聚类结果是因为从我们的角度来看，尾巴几乎是“端上”，而共同的旅行时间只是反映了这些恒星在一个单一的近心通过期间同时从矮星剥离的事实。这个提议是明显可证伪的，因为它做出了许多与黑洞弹射机制不同的预测，并且应该可以用改进的HVS数据集进行测试。
英文摘要：
Halo stars with unusually high radial velocity ("hypervelocity" stars, or HVS) are thought to be stars unbound to the Milky Way that originate from the gravitational interaction of stellar systems with the supermassive black hole at the Galactic center. We examine the latest HVS compilation and find peculiarities that are unexpected in this black hole-ejection scenario. For example, a large fraction of HVS cluster around the constellation of Leo and share a common travel time of $\sim 100$-200 Myr. Furthermore, their velocities are not really extreme if, as suggested by recent galaxy formation models, the Milky Way is embedded within a $2.5\times 10^{12} h^{-1} M_{\odot}$ dark halo with virial velocity of $\sim 220$ km/s. In this case, the escape velocity at $\sim 50$ kpc would be $\sim 600$ km/s and very few HVS would be truly unbound. We use numerical simulations to show that disrupting dwarf galaxies may contribute halo stars with velocities up to and sometimes exceeding the nominal escape speed of the system. These stars are arranged in a thinly-collimated outgoing ``tidal tail'' stripped from the dwarf during its latest pericentric passage. We speculate that some HVS may therefore be tidal debris from a dwarf recently disrupted near the center of the Galaxy. In this interpretation, the angular clustering of HVS results because from our perspective the tail is seen nearly ``end on'', whereas the common travel time simply reflects the fact that these stars were stripped simultaneously from the dwarf during a single pericentric passage. This proposal is eminently falsifiable, since it makes a number of predictions that are distinct from the black-hole ejection mechanism and that should be testable with improved HVS datasets.
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