Intermediate-Mass Black Hole Formation from Hierarchical Mergers in Galactic Nuclei

TL;DR

This study models how hierarchical mergers in galactic nuclei can lead to the formation of intermediate-mass black holes and influence gravitational wave sources, highlighting the importance of initial black hole properties.

Contribution

It introduces a semianalytic model to explore IMBH formation from hierarchical mergers considering initial BH distributions and primordial binaries.

Findings

IMBHs up to ~500 M_sun can form under certain initial conditions

Most IMBHs >200 M_sun sink and merge with the SMBH

Merger rate is >10^{-9} per year per galaxy, affected by initial conditions

Abstract

Dense stellar environments like nuclear star clusters (NSCs) can dynamically assemble gravitational wave (GW) sources. We consider a population of single stellar mass black holes (BHs) in the inner $0.1$~pc of a NSC surrounding a $4 \times 10^6$~M$_\odot$ supermassive black hole (SMBH). Using a semianalytic model, we account for direct collisions between BHs and stars and GW capture between BHs. We explore the effect of the initial BH mass and spin distributions on their final properties and the production of GW sources. Specifically, we consider upper and lower limits for the BH initial mass distribution, and we account for the possibility that a subset of our initial population are the merger products of primordial BH binaries. We find that $\sim 500$ M$_{\odot}$ intermediate mass black holes (IMBHs) can form for our upper limit mass distribution, while our lower limit mass distribution forms none. Most IMBHs $\gtrsim 200$~M$_\odot$ eventually sink towards the center of the cluster and merge with the SMBH. We also find successive BH-star collisions can produce low-spinning BHs with $\chi \lesssim 0.2$. Our results have implications for LIGO-Virgo-KAGRA sources. We find that the overall merger rate depends primarily on the initial BH mass distribution and is $\gtrsim 10^{-9}$~yr$^{-1}$ per galaxy for our range of initial conditions. However, primordial binaries can change the number of second and higher generation mergers by an order of magnitude.