Current stellar evolution models predict a dearth of black holes (BHs) with masses ≳50 M⊙ and ≲5 M⊙, and intermediate-mass black holes (IMBHs; ∼102−105 M⊙) have not yet been detected beyond any reasonable doubt. A natural way to form massive BHs is through repeated mergers, detectable via gravitational wave emission with current LIGO/Virgo or future LISA and ET observations. Nuclear star clusters (NSCs) have masses and densities high enough to retain most of the merger products, which acquire a recoil kick at the moment of merger. We explore the possibility that IMBHs may be born as a result of repeated mergers in NSCs, and show how their formation pathways depend on the NSC mass and density, and BH spin distribution. We find that BHs in the pair-instability mass gap can be formed and observed by LIGO/Virgo, and show that the typical mass of the ejected massive BHs is 400--500 M⊙, with velocities of up to a few thousand km s−1. Eventually some of these IMBHs can become the seeds of supermassive BHs, observed today in the centers of galaxies. In dwarf galaxies, they could potentially solve the abundance, core-cusp, too-big-to-fail, ultra-faint, and baryon-fraction issues via plausible feedback scenarios.
@article{arxiv.2006.01867,
title = {Repeated mergers and ejection of black holes within nuclear star clusters},
author = {Giacomo Fragione and Joseph Silk},
journal= {arXiv preprint arXiv:2006.01867},
year = {2020}
}