Repeated mergers and ejection of black holes within nuclear star clusters

TL;DR

This paper investigates how repeated black hole mergers in nuclear star clusters can produce intermediate-mass black holes, some of which are ejected at high velocities, potentially seeding supermassive black holes and impacting galaxy evolution.

Contribution

It demonstrates the formation of intermediate-mass black holes through repeated mergers in NSCs and analyzes their ejection velocities and observational signatures.

Findings

Black holes in the pair-instability mass gap can form and be detected by LIGO/Virgo.

Ejected massive black holes typically have masses of 400-500 M$_\\odot$ and velocities up to a few thousand km/s.

Some IMBHs may serve as seeds for supermassive black holes in galaxy centers.

Abstract

Current stellar evolution models predict a dearth of black holes (BHs) with masses $\gtrsim 50$ M$_\odot$ and $\lesssim 5$ M$_\odot$, and intermediate-mass black holes (IMBHs; $\sim10^2- 10^5$ M$_\odot$) 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$_\odot$, 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.