The impact of faint AGN discovered by <i>JWST</i> on reionization

Abstract

ABSTRACT The relative contribution of emission from stellar sources and accretion onto supermassive black holes to reionization has been brought into focus again by the apparent high abundance of faint active galactic nuclei (AGNs) at $4\lesssim z\lesssim 11$ uncovered by James Webb Space Telescope (JWST). We investigate here the contribution of these faint AGN to hydrogen and the early stages of helium reionization using the Graphics Processing Unit (GPU) based radiative transfer code aton-he by post-processing a cosmological hydrodynamical simulation from the sherwood-relics suite of simulations. We study four models: two galaxy-only late-end reionization models, a QSO (quasi-stellar object)-assisted, and a QSO-only model. In the QSO-assisted model, 1 per cent of the haloes host AGN, with AGN luminosities scaled to contribute 17 per cent of the total hydrogen-ionizing emissivity. In the QSO-only model, quasars account for all the hydrogen-ionizing emissivity, with 10 per cent of the haloes hosting AGN. The Spectral energy distribution (SED) of AGN is assumed to be a power law with $\alpha =-1.7$ each with a 10 Myr lifetime. All models are calibrated to the observed mean Lyman-$\alpha$ forest transmission at $5\lesssim z\lesssim 6.2$. The QSO-assisted model requires an emissivity similar to the galaxy-only models and fits the observed distribution of the Lyman-$\alpha$ optical depths well. The QSO-only model is inconsistent with the observed Lyman-$\alpha$ optical depths distribution, and produces excessively high intergalactic medium temperatures at $z\lesssim 5$ due to an early onset of He ii reionization, unless the escape fraction of He ii-ionizing photons is assumed to be low. Our results suggest that a modest AGN contribution to reionization aligns with the Lyman-$\alpha$ forest data, whereas an AGN-dominated scenario is difficult to reconcile.

Topics & Concepts