Cosmic Reionization in the JWST Era: Back to AGNs?

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.