UNCOVERing the contribution of black holes to reionization in the JWST era

With its sensitivity in the rest-frame optical, the James Webb Space Telescope (JWST) has uncovered active galactic nuclei (AGN), comprising both intrinsically faint and heavily reddened sources, well into the first billion years of the Universe, at z ∼ 4-11. In this work, we revisit the AGN contribution to reionization given the high number densities associated with these objects. We use the DELPHI semi-analytic model, base-lined against the latest high-redshift datasets from the JWST and the Atacama Large millimetre Array (ALMA) to model early star forming galaxies and AGN. We calculate the escape fractions of ionizing radiation from both star formation and AGN and include the impact of reionization feeback in suppressing the baryonic content of low-mass galaxies in ionized regions. This model is validated against the key observables for star forming galaxy, AGN and reionization. In our fiducial model, reionization reaches its mid-point at z ∼ 6.9 and ends by z ∼ 5.9. Low stellar mass (M_*≤ 10^9M_⊙) star forming galaxies are found to be the key drivers of the reionization process, providing about 77% of the total photon budget. Despite their high numbers, high accretion rates and higher escape fractions compared to star forming galaxies at z ∼ 5, AGN only provide about 23% of the total reionization budget which is dominated by black holes in high stellar mass systems (with M_* ≥ 10^9M_⊙). This is because AGN number densities become relevant only at z ≤ 7 - as a result, AGN contribute as much as galaxies as late as z ∼ 6.2, when reionization is already in its end stages. Finally, we find that even contrasting models of the AGN ionizing photon escape fraction (increasing or decreasing with stellar mass) do not qualitatively change our results.