Growing massive black holes through supercritical accretion of stellar-mass seeds

Abstract

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