The impact of faint AGN discovered by JWST on reionization

TL;DR

This study assesses the role of faint AGN discovered by JWST in the reionization of hydrogen and helium, finding that a modest AGN contribution aligns with observational data, while an AGN-dominated scenario faces challenges.

Contribution

It introduces a detailed analysis of faint AGN's impact on reionization using radiative transfer simulations calibrated to observational data.

Findings

Modest AGN contribution fits Lyman-alpha forest data.

AGN-dominated reionization models conflict with observed IGM temperatures.

QSO-assisted models align well with observations.

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 (AGN) at $4\lesssim z\lesssim11$ uncovered by JWST. We investigate here the contribution of these faint AGN to hydrogen and the early stages of helium reionization using the 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-assisted and a QSO-only model. In the QSO-assisted model, 1% of the haloes host AGN, with AGN luminosities scaled to contribute 17% of the total hydrogen-ionizing emissivity. In the QSO-only model, quasars account for all the hydrogen-ionizing emissivity, with 10% of the haloes hosting AGN. The 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 $5\lesssim z\lesssim6.2$. 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 IGM temperatures at $z\lesssim 5$ due to an early onset of HeII reionization, unless the escape fraction of HeII-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.