Supermassive black hole growth in hierarchically merging nuclear star clusters

TL;DR

This paper explores how supermassive black holes may originate from stellar black hole mergers in nuclear star clusters during early galaxy formation, with implications for observable phenomena and gravitational wave signals.

Contribution

It proposes a new formation pathway for supermassive black hole seeds via stellar black hole mergers in nuclear star clusters at high redshifts.

Findings

Potential observable signatures include intermediate-mass black holes and gravitational wave background.

Ejection of black holes and tidal disruption events are predicted consequences.

Early gas accretion in dwarf galaxies supports black hole growth models.

Abstract

Supermassive black holes are prevalent at the centers of massive galaxies, and their masses scale with galaxy properties, increasing evidence suggesting that these trends continue to low stellar masses. Seeds are needed for supermassive black holes, especially at the highest redshifts explored by the James Webb Space Telescope. We study the hierarchical merging of galaxies via cosmological merger trees and argue that the seeds of supermassive black holes formed in nuclear star clusters via stellar black hole mergers at early epochs. Observable tracers include intermediate-mass black holes, nuclear star clusters, and early gas accretion in host dwarf galaxies, along with a potentially detectable stochastic gravitational wave background, ejection of intermediate and supermassive black holes, and consequences of a significant population of tidal disruption events and extreme-mass ratio inspirals.