Rapid growth of seed black holes in the early universe by supra-exponential accretion

TL;DR

This paper proposes a dynamical mechanism enabling black hole seeds in the early universe to undergo supra-exponential accretion, explaining the rapid formation of supermassive black holes powering quasars within the first billion years.

Contribution

It introduces a new mechanism for rapid black hole growth via supra-exponential accretion triggered by dense gas flows and star cluster dynamics in the early universe.

Findings

Supra-exponential accretion can occur in dense star clusters with trapped radiation.

This mechanism explains the rapid emergence of supermassive black holes by redshift ~6.

Black hole growth can outpace Eddington-limited rates under certain early universe conditions.

Abstract

Mass accretion by black holes (BHs) is typically capped at the Eddington rate, when radiation's push balances gravity's pull. However, even exponential growth at the Eddington-limited e-folding time t_E ~ few x 0.01 billion years, is too slow to grow stellar-mass BH seeds into the supermassive luminous quasars that are observed when the universe is 1 billion years old. We propose a dynamical mechanism that can trigger supra-exponential accretion in the early universe, when a BH seed is trapped in a star cluster fed by the ubiquitous dense cold gas flows. The high gas opacity traps the accretion radiation, while the low-mass BH's random motions suppress the formation of a slowly-draining accretion disk. Supra-exponential growth can thus explain the puzzling emergence of supermassive BHs that power luminous quasars so soon after the Big Bang.