Growing massive black holes through super-critical accretion of stellar-mass seeds

TL;DR

This paper demonstrates through hydrodynamical simulations that stellar-mass black holes can rapidly grow into supermassive black holes via super-critical accretion in high-redshift galaxy centers, explaining early quasar formation.

Contribution

It introduces a novel scenario where stellar-mass black holes undergo super-critical accretion in circum-nuclear discs, leading to rapid growth into supermassive black holes, supported by multi-code hydrodynamical simulations.

Findings

Black holes grow by 3 orders of magnitude within a few million years.

Super-critical accretion is facilitated by dense cold gas reservoirs.

Low radiative efficiency allows rapid growth without excessive heating.

Abstract

The rapid assembly of the massive black holes that power the luminous quasars observed at $z \sim 6-7$ remains a puzzle. Various direct collapse models have been proposed to head-start black hole growth from initial seeds with masses $\sim 10^5\,\rm M_\odot$, which can then reach a billion solar mass while accreting at the Eddington limit. Here we propose an alternative scenario based on radiatively inefficient super-critical accretion of stellar-mass holes embedded in the gaseous circum-nuclear discs (CNDs) expected to exist in the cores of high redshift galaxies. Our sub-pc resolution hydrodynamical simulations show that stellar-mass holes orbiting within the central 100 pc of the CND bind to very high density gas clumps that arise from the fragmentation of the surrounding gas. Owing to the large reservoir of dense cold gas available, a stellar-mass black hole allowed to grow at super-Eddington rates according to the "slim disc" solution can increase its mass by 3 orders of magnitudes within a few million years. These findings are supported by simulations run with two different hydro codes, RAMSES based on the Adaptive Mesh Refinement technique and GIZMO based on a new Lagrangian Godunov-type method, and with similar, but not identical, sub-grid recipes for star formation, supernova feedback, black hole accretion and feedback. The low radiative efficiency of super-critical accretion flows are instrumental to the rapid mass growth of our black holes, as they imply modest radiative heating of the surrounding nuclear environment.