The mass budget for intermediate-mass black holes in dense star clusters
The mass budget for intermediate-mass black holes in dense star clusters
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DOI：
10.1093/mnras/stab1470
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发表时间：
2020-08
影响因子：
4.8
通讯作者：
Yanlong Shi;Michael Y. Grudić;P. Hopkins
中科院分区：
文献类型：
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作者：
Yanlong Shi;Michael Y. Grudić;P. Hopkins
中等质量黑洞（IMBHs）可以通过年轻大质量星团（YMC）中大质量恒星的逃逸合并而形成。我们将一套YMC形成的数值模拟与大质量恒星的动态摩擦和合并以及中心准恒星演化的半解析模型相结合，以预测最终的准恒星和遗迹IMBH质量如何与星团性质相对应（并与观测结果进行比较）。模拟表明，地层内部的YMC密度曲线是陡峭的（接近等温），即使在有效密度相对较低的星团中也能产生一些有效的合并，这与假设中心松弛后的球状星团的平坦中心曲线的模型不同。我们的结果可以用简单的解析标度来近似，$M_{\rm IMBH} \propto v_{\rm cl}^{3/2}$，其中$v_{\rm cl}^{2} = G\, M_{\rm cl}/r_{\rm h}$是用初始星团质量Mcl和半质量半径rh表示的圆速度。虽然这表明即使在典型的星团中也可能形成IMBH，但我们表明这些系统的预测IMBH质量很小，$\sim \! 100-1000\, {\rm M}_{\odot }$或$\sim \! 0.0003\, M_{\rm cl}$，甚至低于所有已知情况下最保守的观测上限。在核星团、超致密矮星或致密椭圆星系的中心，黑洞的质量可以达到$\gtrsim 10^{4}\, {\rm M}_{\odot }$，但在所有这些情况下，预测的质量仍然远远低于目前在这些系统中观测到的超大质量黑洞质量。
Intermediate-mass black holes (IMBHs) could form via runaway merging of massive stars in a young massive star cluster (YMC). We combine a suite of numerical simulations of YMC formation with a semi-analytic model for dynamical friction and merging of massive stars and evolution of a central quasi-star, to predict how final quasi-star and relic IMBH masses scale with cluster properties (and compare with observations). The simulations argue that inner YMC density profiles at formation are steep (approaching isothermal), producing some efficient merging even in clusters with relatively low effective densities, unlike models that assume flat central profiles resembling those of globular clusters after central relaxation. Our results can be approximated by simple analytic scalings, with $M_{\rm IMBH} \propto v_{\rm cl}^{3/2}$ where $v_{\rm cl}^{2} = G\, M_{\rm cl}/r_{\rm h}$ is the circular velocity in terms of initial cluster mass Mcl and half-mass radius rh. While this suggests IMBH formation is possible even in typical clusters, we show that predicted IMBH masses for these systems are small, $\sim \! 100-1000\, {\rm M}_{\odot }$ or $\sim \! 0.0003\, M_{\rm cl}$, below even the most conservative observational upper limits in all known cases. The IMBH mass could reach $\gtrsim 10^{4}\, {\rm M}_{\odot }$ in the centres nuclear star clusters, ultra-compact dwarfs, or compact ellipticals, but in all these cases the prediction remains far below the present observed supermassive BH masses in these systems.