Modelling a bright z = 6 galaxy at the faint end of the AGN luminosity
  function
  Recent deep surveys have unravelled a population of faint active galactic
nuclei (AGN) in the high redshift Universe, leading to various discussions on
their nature and their role during the Epoch of Reionization. We use
cosmological radiation-hydrodynamics simulations of a bright galaxy at z = 6
($M_\star \gtrsim 10^{10} M_\odot$) hosting an actively growing super-massive
black hole to study the properties of these objects. In particular, we study
how the black hole and the galaxy co-evolve and what is the relative
contribution of the AGN and of the stellar populations to the luminosity budget
of the system. We find that the feedback from the AGN has no strong effect on
the properties of the galaxy, and does not increase the total ionizing
luminosity of the host. The average escape fraction of our galaxy is around
$f_{\rm esc} \sim 5\%$. While our galaxy would be selected as an AGN in deep
X-ray surveys, most of the UV luminosity is originating from stellar
populations. This confirms that there is a transition in the galaxy population
from star forming galaxies to quasar hosts, with bright Lyman-Break Galaxies
(LBGs) with $M_{\rm UV}$ around -22 falling in the overlap region. Our results
also suggest that faint AGN do not contribute significantly to reionizing the
Universe.
