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arXiv 1706.04201astro-ph.GA
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AGN外流中超高速星的形成和空间分布

Formation and spatial distribution of hypervelocity stars in AGN outflows

Xiawei Wang, Abraham Loeb

摘要:

我们研究了由活动星系核(AGN)驱动的外流中的恒星形成,作为超高速恒星(HVS)的一个新来源。最近的观测显示,在银河系外流中有活跃的恒星形成,形成速率为$\sim 15\,M_{\odot}\,\rm yr^{-1}$。我们验证了壳层被活动星系核流出物扫过,能够通过热不稳定性冷却并碎裂成嵌入热稀薄气体中的冷团块。我们发现$\sim 10^3\,M_{\odot}$的冷团簇在$\sim 10^5$年内形成,因此,恒星沿着外流路径产生,并在其形成地点被赋予外流速度。这些HVS穿过星系晕,最终逃逸到星系际介质中。流出物内部恒星形成的预期瞬时速率是$\sim 4-5$数量级,大于先前提出的产生HVS的机制的平均速率,例如希尔斯机制和恒星与黑洞双星之间的三体相互作用。我们预测了活动星系核外流中形成的HVS的空间分布,为今后的观测探测提供了依据。

英文摘要:

We study star formation within outflows driven by active galactic nuclei (AGN) as a new source of hypervelocity stars (HVSs). Recent observations revealed active star formation inside a galactic outflow at a rate of $\sim 15\,M_{\odot}\,\rm yr^{-1}$. We verify that the shells swept up by an AGN outflow are capable of cooling and fragmentation into cold clumps embedded in a hot tenuous gas via thermal instabilities. We show that cold clumps of $\sim 10^3\,M_{\odot}$ are formed within $\sim 10^5$ yrs. As a result, stars are produced along outflow's path, endowed with the outflow speed at their formation site. These HVSs travel through the galactic halo and eventually escape into the intergalactic medium. The expected instantaneous rate of star formation inside the outflow is $\sim 4-5$ orders of magnitude greater than the average rate associated with previously proposed mechanisms for producing HVSs, such as the Hills mechanism and three-body interaction between a star and a black hole binary. We predict the spatial distribution of HVSs formed in AGN outflows for future observational probe.

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