The Contribution of AGN Accretion Disks to Hydrogen Reionization

TL;DR

This study assesses the role of high-redshift AGNs in hydrogen reionization, finding their contribution is limited and mostly occurs at later epochs, with implications for SMBH growth and quasar proximity zones.

Contribution

It introduces a model linking SMBH growth, evolving accretion disk spectra, and reionization contribution, considering super-Eddington accretion and matching observed quasar properties.

Findings

AGNs contribute mainly at z<7 to reionization

Evolving SEDs can increase ionizing radiation by ~80%

Late super-Eddington accretion explains proximity zones

Abstract

We examine the contribution of high-redshift (z>6) active galactic nuclei (AGNs) to cosmic hydrogen reionization, by tracing the growth and ionizing output of the first generation of supermassive black holes (SMBHs). Our calculations are anchored to the observed population of z~6 quasars, and trace back the evolving spectral energy distributions (SEDs) of the accretion flows that power these early AGNs and consider a variety of growth histories, including super-Eddington accretion. Compared to a fixed-shape SED, the evolving thin disks can produce ionizing radiation that is higher by up to ~80%. Across a variety of SMBH growth scenarios, the contribution of AGNs to reionization is limited to late epochs (z<7), and remains sub-dominant compared to star-forming galaxies. This conclusion holds irrespective of the (still unknown) space density of low-luminosity z=6 AGNs, and for growth scenarios that allow super-Eddington accretion. The contribution of AGNs to reionization can extend to earlier epochs (z>8) in scenarios with relative slow SMBH mass growth, i.e., for low accretion rates and/or high spins. We finally demonstrate that our framework can reproduce the observed quasar proximity zone sizes, and that compact proximity zones around z=6 quasars can be explained by the late onset of super-Eddington accretion.