UNCOVERing the contribution of black holes to reionization

Context. With its sensitivity in the rest-frame optical, the James Webb Space Telescope (JWST) has uncovered active galactic nuclei (AGN), which comprise intrinsically faint and heavily reddened sources, well into the first billion years of the Universe, at z ∼ 4-11. Aims. We revisit the AGN contribution to reionization given the high number densities associated with these objects. Methods. We used the DELPHI semi-analytic model, which we 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 calculated the escape fractions of ionizing radiation from star formation and AGN and included the impact of reionization feeback in suppressing the baryonic content of low-mass galaxies in ionized regions. This model was validated against the key observables for star-forming galaxies, AGN, and reionization. Results. In our fiducial model, reionization reaches its mid-point at z ∼ 6.9 and ends by z ∼ 5.9. Low stellar mass (M_∗ ≤ 10⁹ M_⊙) star-forming galaxies are found to be the key drivers of the reionization process. They provide about 77% of the total photon budget. Despite their high numbers, high accretion rates, and higher escape fractions than 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_∗ ≥ 10⁹ M_⊙). This is because AGN number densities become relevant only at z ≤ 7, and 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.