Several binary black holes (BBHs) have been observed using gravitational wave detectors. For the formation mechanism of BBHs, two main mechanisms, isolated binary evolution and dynamical formation in dense star clusters, have been suggested. Future observations are expected to provide more information about BBH distributions, and it will help us to distinguish the two formation mechanisms. For the star cluster channel, globular clusters have mainly been investigated. However, recent simulations have suggested that BBH formation in open clusters is not negligible. We estimate a local merger rate density of BBHs originated from open clusters using the results of our $N$-body simulations of open clusters with four different metallicities. We find that the merger rate per cluster is the highest for our 0.1 solar metallicity model. Assuming a cosmic star formation history and a metallicity evolution with dispersion, we estimate the local merger rate density of BBHs originated from open clusters to be $\sim 70~{\rm yr}^{-1} {\rm Gpc}^{-3}$. This value is comparable to the merger rate density expected from the first and second observation runs of LIGO and Virgo. In addition, we find that BBH mergers obtained from our simulations can reproduce the distribution of primary mass and mass ratio of merging BBHs estimated from the LIGO and Virgo observations.

中文翻译：

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疏散星团中形成的二元黑洞的合并率密度

已经使用引力波探测器观测到了几个双黑洞 (BBH)。对于BBHs的形成机制，已经提出了两种主要机制，孤立的双星演化和致密星团中的动力学形成。预计未来的观测将提供更多关于 BBH 分布的信息，这将有助于我们区分两种形成机制。对于星团通道，主要研究的是球状星团。然而，最近的模拟表明，疏散星团中的 BBH 形成是不可忽略的。我们使用具有四种不同金属丰度的疏散星团的 $N$-body 模拟结果估计源自疏散星团的 BBH 的局部合并率密度。我们发现对于我们的 0.1 太阳金属丰度模型，每个集群的合并率是最高的。假设宇宙恒星形成历史和金属丰度随色散演化，我们估计源自疏散星团的 BBH 的局部合并率密度为 $\sim 70~{\rm yr}^{-1} {\rm Gpc}^{ -3}$。该值与 LIGO 和 Virgo 的第一次和第二次观测运行所预期的合并率密度相当。此外，我们发现从我们的模拟中获得的 BBH 合并可以重现从 LIGO 和 Virgo 观测估计的合并 BBH 的初级质量和质量比的分布。