Hyper-Eddington accretion flows onto massive black holes

TL;DR

This paper demonstrates that massive black holes can undergo hyper-Eddington accretion in dense, low-metallicity environments, leading to rapid growth beyond traditional limits, with potential observable signatures.

Contribution

It introduces a new regime of steady, hyper-Eddington accretion onto black holes, characterized by photon trapping and a two-part flow structure, expanding understanding of black hole growth in early galaxies.

Findings

Accretion rates can exceed 3000 times the Eddington rate under certain conditions.

Steady hyper-Eddington accretion occurs when ambient density and temperature satisfy specific criteria.

Potential observational signatures include Lyman-alpha emitters without X-ray counterparts.

Abstract

We study very-high rate spherically symmetric accretion flows onto a massive black hole (BH; 10^2 < M_BH < 10^6 Msun) embedded in a dense gas cloud with a low abundance of metals, performing one-dimensional hydrodynamical simulations which include multi-frequency radiation transfer and non-equilibrium primordial chemistry. We find that rapid gas supply from the Bondi radius at a hyper-Eddington rate can occur without being impeded by radiation feedback when (n/10^5 cm^-3) > (M_BH/10^4Msun)^{-1}(T/10^4 K)^{3/2}, where n and T are the density and temperature of ambient gas outside of the Bondi radius. The resulting accretion rate in this regime is steady, and larger than 3000 times the Eddington rate. At lower Bondi rates, the accretion is episodic due to radiative feedback and the average rate is limited below the Eddington rate. For the hyper-Eddington case, the steady solution consists of two parts: a radiation-dominated central core, where photon trapping due to electron scattering is important, and an accreting envelope which follows a Bondi profile with T~8000 K. When the emergent luminosity is limited below the Eddington luminosity because of photon trapping, radiation from the central region does not affect the gas dynamics at larger scales. We apply our result to the rapid formation of massive BHs in protogalaxies with a virial temperature of T_vir> 10^4 K. Once a seed BH forms at the center of the galaxy, it can grow up to a maximum ~10^5 (T_vir/10^4 K) Msun via gas accretion independent of the initial BH mass. Finally, we discuss possible observational signatures of rapidly accreting BHs with/without allowance for dust. We suggest that these systems could explain Lya emitters without X-rays and luminous infrared sources with hot dust emission, respectively.