Hierarchical Merger of Primordial Black Holes in Dwarf Galaxies

TL;DR

This paper explores how primordial black holes in dwarf galaxy cores can undergo hierarchical mergers, leading to more massive black holes and significant gravitational wave emission, with implications for dark matter and black hole evolution.

Contribution

It demonstrates that hierarchical mergers of primordial black holes can produce substantially more massive black holes within a Hubble time, transferring about 40% of the core mass.

Findings

Up to 40% of core mass is transferred to merger products.

Approximately 5% of energy is emitted as gravitational waves.

Black holes up to eight times more massive can form within a Hubble time.

Abstract

We study the merger history of primordial black holes (PBHs) in a scenario where they represent the dominant dark matter component of a typical dwarf galaxies' core. We investigate the possibility of a sequence of collisions resulting in a hierarchical merger of black holes and look at the final mass spectrum in such {\it clusters}, which initially present a monochromatic (single-mass) PBH population. Our study shows that the merging process results in the transfer of about $40\%$ of the total mass of the core to the merger products regardless of the initial mass of PBHs, with about $5\%$ of energy radiated out in the form of gravitational waves. We find that, in the lighter mass limit, black holes up to eight times more massive than the original population can be formed within a Hubble time.