Sowing Black Hole Seeds: Direct Collapse Black Hole Formation with Realistic Lyman–Werner Radiation in Cosmological Simulations

Abstract

Abstract We study the birth of supermassive black holes from the direct collapse process and characterize the sites where these black hole seeds form. In the pre-reionization epoch, molecular hydrogen (H2) is an efficient coolant, causing gas to fragment and form Population III stars, but Lyman–Werner radiation can suppress H2 formation and allow gas to collapse directly into a massive black hole. The critical flux required to inhibit H2 formation, J crit, is hotly debated, largely due to the uncertainties in the source radiation spectrum, H2 self-shielding, and collisional dissociation rates. Here, we test the power of the direct collapse model in a self-consistent, time-dependant, nonuniform Lyman–Werner radiation field—the first time such has been done in a cosmological volume—using an updated version of the Smooth Particle Hydrodynamics (SPH)+N-body tree code Gasoline with H2 nonequilibrium abundance tracking, H2 cooling, and a modern SPH implementation. We vary J crit from 30 to 103 in units of J 21 to study how this parameter impacts the number of seed black holes and the type of galaxies that host them. We focus on black hole formation as a function of environment, halo mass, metallicity, and proximity of the Lyman–Werner source. Massive black hole seeds form more abundantly with lower J crit thresholds, but regardless of J crit, these seeds typically form in halos that have recently begun star formation. Our results do not confirm the proposed atomic cooling halo pair scenario; rather, black hole seeds predominantly form in low-metallicity pockets of halos that already host star formation.