Intermediate-mass black holes from Population III remnants in the first galactic nuclei

TL;DR

This study uses numerical simulations to demonstrate that Population III remnant black holes can grow into intermediate-mass black holes in early protogalaxies, potentially explaining the formation of the earliest massive black holes and their gravitational wave signatures.

Contribution

It introduces a new simulation-based pathway for IMBH formation from Population III remnants, including hyper-Eddington accretion effects in gas-rich early galaxies.

Findings

Most initial BH configurations lead to a single central IMBH.

IMBHs can reach masses of 10^3-10^5 solar masses.

Captured stellar-mass BH companions could be observable as EMRIs in gravitational waves.

Abstract

We report the formation of intermediate-mass black holes (IMBHs) in suites of numerical $N$-body simulations of Population III remnant black holes (BHs) embedded in gas-rich protogalaxies at redshifts $z\gtrsim10$. We model the effects of gas drag on the BHs' orbits, and allow BHs to grow via gas accretion, including a mode of hyper-Eddington accretion in which photon trapping and rapid gas inflow suppress any negative radiative feedback. Most initial BH configurations lead to the formation of one (but never more than one) IMBH in the center of the protogalaxy, reaching a mass of $10^{3-5}\mathrm{M}_{\odot}$ through hyper-Eddington growth. Our results suggest a viable pathway to forming the earliest massive BHs in the centers of early galaxies. We also find that the nuclear IMBH typically captures a stellar-mass BH companion, making these systems observable in gravitational waves as extreme mass-ratio inspirals (EMRIs) with \textit{eLISA}.