Accelerated growth of seed black holes by dust in the early universe

Abstract

We explore the effect of dust on the growth of seed black holes (BHs) in the early universe. Previous 1D radiation-hydrodynamic (RHD) simulations show that increased radiation pressure on dust further suppresses the accretion rate than the case for the chemically pristine gas. Using the Enzo+Moray code, we perform a suite of 3D RHD simulations of accreting BHs in a dusty interstellar medium (ISM). We use the modified Grackle cooling library to consider dust physics in its non-equilibrium chemistry. The BH goes through an early evolutionary phase, where ionizing BH radiation creates an oscillating HII region as it cycles between accretion and feedback. As the simulations proceed, dense cold gas accumulates outside the ionized region where inflow from the neutral medium meets the outflow driven by radiation pressure. In the late phase, high-density gas streams develop and break the quasi-spherical symmetry of the ionized region, rapidly boosting the accretion rate. The late phase is characterized by the coexistence of strong ionized outflows and fueling high-density gas inflows. The mean accretion rate increases with metallicity reaching a peak at Z$\sim$0.01-0.1$\,Z_\odot$, one order of magnitude higher than the one for pristine gas. However, as the metallicity approaches the solar abundance, the mean accretion rate drops as the radiation pressure becomes strong enough to drive out the high-density gas. Our results indicate that a dusty metal-poor ISM can accelerate the growth rate of BHs in the early universe, however, can stun its growth as the ISM is further enriched toward the solar abundance.