paper

Growing Black Holes through Successive Mergers in Galactic Nuclei: I. Methods and First Results

arXiv:2211.09670 · doi:10.1093/mnras/stad1634

Abstract

We present a novel, few-body computational framework designed to shed light on the likelihood of forming intermediate-mass (IM) and supermassive (SM) black holes (BHs) in nuclear star clusters (NSCs) through successive BH mergers, initiated with a single BH seed. Using observationally motivated NSC profiles, we find that the probability of a BH to grow beyond through successive mergers ranges from in low-density, low-mass clusters to nearly in high-mass, high-density clusters. However, in the most massive NSCs, the growth timescale can be very long (Gyr); vice versa, while growth is least likely in less massive NSCs, it is faster there, requiring as little as Gyr. The increased gravitational focusing in systems with lower velocity dispersions is the primary contributor to this behavior. We find that there is a simple "7-strikes-and-you're-in" rule governing the growth of BHs: our results suggest that if the seed survives 7 to 10 successive mergers without being ejected (primarily through gravitational wave recoil kicks), the growing BH will most likely remain in the cluster and will then undergo runaway, continuous growth all the way to the formation of an SMBH (under the simplifying assumption adopted here of a fixed background NSC). Furthermore, we find that rapid mergers enforce a dynamically-mediated "mass gap" between about in an NSC.

arXiv comment26 pages, 21 figures, and 3 tables. Submitted to MNRAS. Comments welcome

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