Astraeus VI: Hierarchical assembly of AGN and their large-scale effect during the Epoch of Reionization

In this work, the sixth of a series, we use the ASTRAEUS (semi-numerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dark matter simUlationS) framework to investigate the nature of the sources that reionized the Universe. We extend ASTRAEUS, which already couples a galaxy formation semi-analytical model with a detailed semi-numerical reionization scheme, to include a model for black hole formation, growth, and the production of ionizing radiation from associated AGN (active galactic nuclei). We calibrate our fiducial AGN model to reproduce the bolometric luminosity function at z ~ 5, and explore the role of the resulting AGN population in reionizing the Universe. We find that in all the models yielding a reasonable AGN luminosity function, galaxies dominate overwhelmingly the ionizing budget during the Epoch of Reionization, with AGN accounting for 1-10% of the ionizing budget at z=6 and starting to play a role only below z < 5.