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.
