Accretion onto Seed Black Holes in the First Galaxies

TL;DR

This paper investigates how radiation feedback from black holes in early galaxies reduces accretion rates, challenging the assumption that seed black holes grow rapidly via quasiradial, Eddington-limited accretion in metal-poor environments.

Contribution

It demonstrates that radiation pressure significantly diminishes accretion efficiency, indicating the need for alternative growth mechanisms for seed black holes in early galaxies.

Findings

Radiation pressure from photoionization reduces accretion rates.

Steady-state accretion is a small fraction of the Eddington rate.

Ly-alpha photon trapping may further inhibit inflow.

Abstract

The validity of the hypothesis that the massive black holes in high redshift quasars grew from stellar-sized "seeds" is contingent on a seed's ability to double its mass every few ten million years. This requires that the seed accrete at approximately the Eddington-limited rate. In the specific case of radiatively efficient quasiradial accretion in a metal-poor protogalactic medium, for which the Bondi accretion rate is often prescribed in cosmological simulations of massive black hole formation, we examine the effects of the radiation emitted near the black hole's event horizon on the structure of the surrounding gas flow. We find that the radiation pressure from photoionization significantly reduces the steady-state accretion rate and renders the quasiradial accretion flow unsteady and inefficient. The time-averaged accretion rates are a small fraction of the Eddington-limited accretion rate for Thomson scattering. The pressure of Ly-alpha photons trapped near the HII region surrounding the black hole may further attenuate the inflow. These results suggest that an alternative to quasiradial, radiatively efficient Bondi-like accretion should be sought to explain the rapid growth of quasar-progenitor seed black holes.