Super-Eddington accretion of dusty gas on to seed black holes: metallicity-dependent efficiency of mass growth.

The super-Eddington accretion on to intermediate seed black holes (BHs) is a potential formation mode of supermassive black holes exceeding |$10^9\, \mathrm{ M}_\odot$| in the early universe. We here investigate how such rapid accretion may occur with finite amounts of heavy elements contained in the gas and dust. In our 1D radiation-hydrodynamics simulations, the radiative transfer is solved for both the direct ultraviolet lights emitted by an accretion disc and the diffuse infrared (IR) lights thermally emitted by dust grains. Our results show that the radiative force by the IR lights causes a strong feedback to regulate the mass accretion. The resulting mean accretion rate is lower with the higher metallicity, and there is the critical metallicity Z ∼ 10⁻² Z_⊙, above which the super-Eddington accretion is prevented by the radiation pressure of the IR lights. With this taken into account, we examine if the dusty super-Eddington accretion occurs in young galaxies using a simple model. We show that a sufficient number of galaxies at |$z$| ≳ 10 can be such potential sites if BHs accrete the cold dense gas with T ∼ 10² K, approximately the thermal equilibrium value at Z  = 10⁻² Z_⊙. We argue that the efficiency of the BH growth via the rapid accretion depends on the metallicity, and that the metallicity slightly lower than |$10^{-2}\, \mathrm{ Z}_\odot$| provides a chance for the most efficient growth. [ABSTRACT FROM AUTHOR]