Cosmic Reionization in the JWST Era: Back to AGNs?

TL;DR

This paper reevaluates the role of AGNs in cosmic reionization using JWST data, suggesting they could be the primary drivers of hydrogen and helium reionization with high escape fractions and consistent observational constraints.

Contribution

It presents a new model indicating AGNs alone can account for reionization, aligning with recent JWST observations and existing cosmological constraints.

Findings

Hydrogen reionization completed by z~5.3-5.5.

Thomson optical depth ~0.05, consistent with CMB data.

AGN population does not violate X-ray background constraints.

Abstract

Deep surveys with the James Webb Space Telescope (JWST) have revealed an emergent population of moderate-luminosity, broad-line active galactic nuclei (AGNs) at 4< z< 14 powered by accretion onto early massive black holes. The high number densities reported, together with the large Lyman-continuum (LyC) production efficiency and leakiness into the intergalactic medium (IGM) that are typical of UV-selected AGNs, lead us to reassess a scenario where AGNs are the sole drivers of the cosmic hydrogen/helium reionization process. Our approach is based on the assumptions, grounded in recent observations, that: (a) the fraction of broad-line AGNs among galaxies is around 10-15%; (b) the mean escape fraction of hydrogen LyC radiation is high, >80%, in AGN hosts and is negligible otherwise; and (c) internal absorption at 4 ryd or a steep ionizing EUV spectrum delay full reionization of HeII until z~2.8-3.0, in agreement with observations of the HeII Lyman-alpha forest. In our fiducial models: 1) hydrogen reionization is 99% completed by redshift z~5.3-5.5, and reaches its midpoint at z~6.5-6.7; (2) the integrated Thomson scattering optical depth to reionization is ~0.05, consistent with constraints from cosmic microwave background (CMB) anisotropy data; and (3) the abundant AGN population detected by JWST does not violate constraints on the unresolved X-ray background.