The impact of reionization on the formation of supermassive black hole seeds

TL;DR

This study uses cosmological simulations to investigate how ionizing radiation influences the formation of supermassive black hole seeds via direct collapse, highlighting the role of self-shielding in enabling or preventing this process.

Contribution

It provides the first detailed simulation analysis of the combined effects of Lyman-Werner and ionizing radiation on primordial gas collapse for DCBH formation.

Findings

Ionizing radiation can delay gas collapse by ~25 Myr if not self-shielded.

When ionized gas collapses, H2 formation is catalyzed, preventing DCBH formation.

Self-shielding allows some haloes to avoid suppression of black hole seed formation.

Abstract

Direct collapse black holes (DCBHs) formed from the collapse of atomically-cooled primordial gas in the early Universe are strong candidates for the seeds of supermassive BHs. DCBHs are thought to form in atomic cooling haloes in the presence of a strong molecule-dissociating, Lyman-Werner (LW) radiation field. Given that star forming galaxies are likely to be the source of the LW radiation in this scenario, ionizing radiation from these galaxies may accompany the LW radiation. We present cosmological simulations resolving the collapse of primordial gas into an atomic cooling halo, including the effects of both LW and ionizing radiation. We find that in cases where the gas is not self-shielded from the ionizing radiation, the collapse can be delayed by ~ 25 Myr. When the ionized gas does collapse, the free electrons that are present catalyze H2 formation. In turn, H2 cooling becomes efficient in the center of the halo, and DCBH formation is prevented. We emphasize, however, that in many cases the gas collapsing into atomic cooling haloes at high redshift is self-shielding to ionizing radiation. Therefore, it is only in a fraction of such haloes in which DCBH formation is prevented due to reionization.