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Arxiv: UNCOVERing the contribution of black holes to reionization in the JWST era Published: 1/20/2024 4:50:22 PM Updated: 1/20/2024 4:50:22 PM

Paper abstract: With its sensitivity in the rest-frame optical, the James Webb SpaceTelescope (JWST) has uncovered active galactic nuclei (AGN), comprising bothintrinsically faint and heavily reddened sources, well into the first billionyears of the Universe, at z ~ 4-11. In this work, we revisit the AGNcontribution to reionization given the high number densities associated withthese objects. We use the DELPHI semi-analytic model, base-lined against thelatest high-redshift datasets from the JWST and the Atacama Large millimetreArray (ALMA) to model early star forming galaxies and AGN. We calculate theescape fractions of ionizing radiation from both star formation and AGN andinclude the impact of reionization feeback in suppressing the baryonic contentof low-mass galaxies in ionized regions. This model is validated against thekey observables for star forming galaxy, AGN and reionization. In our {\itfiducial} model, reionization reaches its mid-point at z ~ 6.9 and ends byz ~ 5.9. Low stellar mass (M_*<= 10^9M_\odot) star forming galaxiesare found to be the key drivers of the reionization process, providing about77\% of the total photon budget. Despite their high numbers, high accretionrates and higher escape fractions compared to star forming galaxies at z ~5, AGN only provide about 23\% of the total reionization budget which isdominated by black holes in high stellar mass systems (with M_* >=10^9M_\odot). 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 thateven contrasting models of the AGN ionizing photon escape fraction (increasingor decreasing with stellar mass) do not qualitatively change our results.