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arXiv 1803.00568astro-ph.HEastro-ph.GA
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先进LIGO模型中中等质量黑洞逃逸增长的极限

Limits on Runaway Growth of Intermediate Mass Black Holes from Advanced LIGO

Ely D. Kovetz, Ilias Cholis, Marc Kamionkowski, Joseph Silk

摘要:

越来越多的证据表明,中等质量黑洞(IMBH)(这里定义为质量在M=500-10^5 Msun之间)存在于某些球状星团(GC)的致密中心。它们与其他IMBH或星团中的恒星BH合并产生的引力波(GW)大多以<10 Hz的频率发射,不幸的是,这是目前地面天文台如高级LIGO(aLIGO)无法达到的。尽管如此,我们表明,aLIGO测量可以用来有效地探测一个可能的形成机制的IMBHs在GC中,即一个失控的合并过程中的恒星种子BH。在这种情况下,aLIGO将对合并阶梯的较低质量梯级敏感,范围从种子BH质量到质量>~50-300 Msun,其中来自标准合并的背景预计非常低。假设这个通用的IMBH形成的情况下,我们计算的质量函数,对应于可能的合并树的限制情况。基于对GCs数密度的估计,并考虑到仪器的灵敏度,我们发现目前的观测并不能有效地限制由GCs中恒星黑洞的失控合并所形成的IMBHs的占据分数f_occ。然而,我们发现,在设计灵敏度下运行六年的aLIGO将能够在99.9%的置信水平下探测到f_occ<3%,或者找到这种形成机制的证据,或者如果含有IMBH的GC的比例更高,则需要其他证据。

英文摘要:

There is growing evidence that intermediate-mass black holes (IMBHs), defined here as having a mass in the range M=500-10^5 Msun, are present in the dense centers of certain globular clusters (GCs). Gravitational waves (GWs) from their mergers with other IMBHs or with stellar BHs in the cluster are mostly emitted in frequencies <10 Hz, which unfortunately is out of reach for current ground-based observatories such as advanced LIGO (aLIGO). Nevertheless, we show that aLIGO measurements can be used to efficiently probe one of the possible formation mechanisms of IMBHs in GCs, namely a runaway merger process of stellar seed BHs. In this case, aLIGO will be sensitive to the lower-mass rungs of the merger ladder, ranging from the seed BH mass to masses >~50-300 Msun, where the background from standard mergers is expected to be very low. Assuming this generic IMBH formation scenario, we calculate the mass functions that correspond to the limiting cases of possible merger trees. Based on estimates for the number density of GCs and taking into account the instrumental sensitivity, we show that current observations do not effectively limit the occupation fraction f_occ of IMBHs formed by runaway mergers of stellar BHs in GCs. However, we find that a six-year run of aLIGO at design sensitivity will be able to probe down to f_occ<3% at a 99.9% confidence level, either finding evidence for this formation mechanism, or necessitating others if the fraction of GCs that harbor IMBHs is higher.

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