Early Cosmic Merger of Multiple Black Holes
Early Cosmic Merger of Multiple Black Holes
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DOI：
10.1093/mnras/stv1099
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发表时间：
2015-05
期刊：
影响因子：
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通讯作者：
H. Tagawa;M. Umemura;N. Gouda;T. Yano;Yuki Yamai
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文献类型：
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作者：
H. Tagawa;M. Umemura;N. Gouda;T. Yano;Yuki Yamai
我们对宇宙早期原始气体中多个黑洞（BHs）的合并进行了数值模拟。我们考虑了两种黑洞质量的情况：$M_{BH} = 30M_{\odot}$和$M_{BH} = 10^4M_{\odot}$。重点关注宿主物体中气体动力摩擦的影响。模拟包含了诸如近心点偏移和引力波发射等广义相对论效应。结果，我们发现在很宽的黑洞密度范围内，多个黑洞能够在1亿年内合并为一个黑洞。合并机制被揭示分为三种类型：气体拖曳驱动合并（A类）、相互作用驱动合并（B类）和三体驱动合并（C类）。我们发现了合并机制与初始黑洞轨道内气体质量（$M_{gas}$）和总黑洞质量（${\Sigma}M_{BH}$）之比的关系。如果$M_{gas} \gtrsim 10^5 {\Sigma}M_{BH}$，则发生A类合并；如果$M_{gas} \lesssim 10^5 {\Sigma}M_{BH}$，则为B类；如果$M_{gas} \ll 10^5 {\Sigma}M_{BH}$，则为C类。根据近期关于第一代恒星的数值模拟假设气体和黑洞的密度，第一代恒星产生的所有黑洞遗迹可能通过B类或C类机制合并为一个黑洞。此外，我们发现，如果气体密度高于$5\times 10^6$ cm$^{-3}$，分布在几个秒差距范围内的多个大质量黑洞（$M_{BH} = 10^4M_{\odot}$）能够通过B类机制合并为一个黑洞。目前的结果意味着黑洞合并可能对高红移时期超大质量黑洞的形成有重要贡献。
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 (${\Sigma}M_{BH}$). Type A merger occurs if $M_{gas} \gtrsim 10^5 {\Sigma}M_{BH}$, type B if $M_{gas} \lesssim 10^5 {\Sigma}M_{BH}$, and type C if $M_{gas} \ll 10^5 {\Sigma}M_{BH}$. 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.