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arXiv 1606.01909astro-ph.GAastro-ph.HEgr-qc
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银河系核中大质量黑洞的形成:失控潮汐碰撞

Formation of Massive Black Holes in Galactic Nuclei: Runaway Tidal Encounters

Nicholas C. Stone, Andreas H. W. Kuepper, Jeremiah P. Ostriker

摘要:

核星团(NSC)和超大质量黑洞(SMBH)都居住在星系核中,共存于一系列的隆起质量中,但在最大或最小的星系中相互排斥。我们建议,一旦神经干细胞超过一定的临界中心密度和速度分散,通过失控的潮汐捕获发生的神经干细胞转化为SMBHs。这个过程中的瓶颈,就像所有的碰撞逃逸一样,是黑洞质量的第一个e-折叠。恒星质量的增长黑洞过去这个瓶颈发生的潮汐捕获恒星消耗在重复事件的质量转移在近心。一旦黑洞达到100-1000太阳质量,潮汐捕获可能会关闭作为生长通道,但潮汐破坏事件将继续,并出现能够生长种子SMBH到更大的尺寸。一旦种子SMBH消耗其宿主NSC的核心,失控就会减慢(变为亚指数)。虽然SMBH中的大部分宇宙质量密度最终是由非常大质量星系中的偶发气体吸积产生的(通过Soltan-Paczynski论点),但最小的SMBH可能是从与NSC恒星的强烈潮汐相遇中成长起来的。SMBH种子在一段时间内完全通过这个通道生长$t$,将遵循与宿主星系速度色散$σ$的简单幂律关系。在最简单的情况下,它是$M_\bullet \sim σ^{3/2}\sqrt{M_\star t / G} \sim 10^{6}M_\odot (σ/ 50~{\rm km~s}^{-1})^{3/2}(t/10^{10}~{\rm yr})^{1/2}$,但是指数和前因子可以根据损失锥再填充的细节而略有不同。目前的潮汐中断事件率预测从这个机制是一致的观测。

英文摘要:

Nuclear star clusters (NSCs) and supermassive black holes (SMBHs) both inhabit galactic nuclei, coexisting in a range of bulge masses, but excluding each other in the largest or smallest galaxies. We propose that the transformation of NSCs into SMBHs occurs via runaway tidal captures, once NSCs exceed a certain critical central density and velocity dispersion. The bottleneck in this process, as with all collisional runaways, is growing the first e-fold in black hole mass. The growth of a stellar mass black hole past this bottleneck occurs as tidally captured stars are consumed in repeated episodes of mass transfer at pericenter. Tidal captures may turn off as a growth channel once the black hole reaches a mass ~100-1000 solar masses, but tidal disruption events will continue and appear capable of growing the seed SMBH to larger sizes. The runaway slows (becomes sub-exponential) once the seed SMBH consumes the core of its host NSC. While the bulk of the cosmic mass density in SMBHs is ultimately produced (via the Soltan-Paczynski argument) by episodic gaseous accretion in very massive galaxies, the smallest SMBHs have probably grown from strong tidal encounters with NSC stars. SMBH seeds that grow for a time $t$ entirely through this channel will follow simple power law relations with the velocity dispersion, $σ$, of their host galaxy. In the simplest regime it is $M_\bullet \sim σ^{3/2}\sqrt{M_\star t / G} \sim 10^{6}M_\odot (σ/ 50~{\rm km~s}^{-1})^{3/2}(t/10^{10}~{\rm yr})^{1/2}$, but the exponents and prefactor can differ slightly depending on the details of loss cone refilling. Current tidal disruption event rates predicted from this mechanism are consistent with observations.

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