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arXiv 1505.03151astro-ph.GAastro-ph.HE
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多个黑洞的早期宇宙合并

Early Cosmic Merger of Multiple Black Holes

Hiromichi Tagawa, Masayuki Umemura, Naoteru Gouda, Taihei Yano, Yuki Yamai

备注：
12 pages, 7 figures, accepted for publication in MNRAS
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摘要：

本文对宇宙早期原初气体中的多黑洞并合进行了数值模拟。我们考虑两例BH肿块：$M_{BH} = 30 M_{\odot}$和$M_{BH} = 10^4 M_{\odot}$。注意力集中在主对象中的气体的动态摩擦的效果。模拟纳入了诸如近心点位移和引力波发射等广义相对论效应。因此，我们发现，多个BH能够合并成一个BH在1亿年内在一个广泛的BH密度。发现并合机制可分为三种类型：气体拖曳驱动的并合（A型）、相互作用驱动的并合（B型）和三体驱动的并合（C型）。我们找到了初始BH轨道内气体质量（$M_{gas}$）与BH总质量（$ΣM_{BH}$）之比与合并机制之间的关系。如果是$M_{gas} \gtrsim 10^5 ΣM_{BH}$，则发生A类合并;如果是$M_{gas} \lesssim 10^5 ΣM_{BH}$，则发生B类合并;如果是$M_{gas} \ll 10^5 ΣM_{BH}$，则发生C类合并。假设气体和黑洞的密度是基于最近对第一恒星的数值模拟，那么所有来自第一恒星的黑洞残余都可能通过B或C型机制合并成一个黑洞。此外，我们还发现当气体密度大于$^{-3}$ $5\times 10^6$ cm时，分布在数秒差距内的多个大质量黑洞（$M_{BH} = 10^4 M_{\odot}$）可以通过B型机制合并成一个黑洞.这些结果表明，黑洞并合可能对高红移时期超大质量黑洞的形成有重要贡献。

英文摘要：

We perform numerical simulations on the merger of multiple black holes (BHs) in primordial gas at early cosmic epochs. We consider two cases of BH mass: $M_{BH} = 30 M_{\odot}$ and $M_{BH} = 10^4 M_{\odot}$. Attention is concentrated on the effect of the dynamical friction by gas in a host object. The simulations incorporate such general relativistic effects as the pericentre shift and gravitational wave emission. As a result, we find that multiple BHs are able to merge into one BH within 100 Myr in a wide range of BH density. The merger mechanism is revealed to be categorized into three types: gas-drag-driven merger (type A), interplay-driven merger (type B), and three-body-driven merger (type C). We find the relation between the merger mechanism and the ratio of the gas mass within the initial BH orbit ($M_{gas}$) to the total BH mass ($ΣM_{BH}$). Type A merger occurs if $M_{gas} \gtrsim 10^5 ΣM_{BH}$, type B if $M_{gas} \lesssim 10^5 ΣM_{BH}$, and type C if $M_{gas} \ll 10^5 ΣM_{BH}$. Supposing the gas and BH density based on the recent numerical simulations on first stars, all the BH remnants from first stars are likely to merge into one BH through the type B or C mechanism. Also, we find that multiple massive BHs ($M_{BH} = 10^4 M_{\odot}$) distributed over several parsec can merge into one BH through the type B mechanism, if the gas density is higher than $5\times 10^6$ cm$^{-3}$. The present results imply that the BH merger may contribute significantly to the formation of supermassive BHs at high redshift epochs.

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