Dark-ages Reionization and Galaxy Formation Simulation - X. The small
  contribution of quasars to reionization
  Motivated by recent measurements of the number density of faint AGN at high
redshift, we investigate the contribution of quasars to reionization by
tracking the growth of central supermassive black holes in an update of the
Meraxes semi-analytic model. The model is calibrated against the observed
stellar mass function at $z\sim0.6-7$, the black hole mass function at
$z\lesssim0.5$, the global ionizing emissivity at $z\sim2-5$ and the Thomson
scattering optical depth. The model reproduces a Magorrian relation in
agreement with observations at $z<0.5$ and predicts a decreasing black hole
mass towards higher redshifts at fixed total stellar mass. With the
implementation of an opening angle of 80 deg for quasar radiation,
corresponding to an observable fraction of ${\sim}23.4$ per cent due to
obscuration by dust, the model is able to reproduce the observed quasar
luminosity function at $z\sim0.6-6$. The stellar light from galaxies hosting
faint AGN contributes a significant or dominant fraction of the UV flux. At
high redshift, the model is consistent with the bright end quasar luminosity
function and suggests that the recent faint $z\sim4$ AGN sample compiled by
Giallongo et al. (2015) includes a significant fraction of stellar light.
Direct application of this luminosity function to the calculation of AGN
ionizing emissivity consequently overestimates the number of ionizing photons
produced by quasars by a factor of 3 at $z\sim6$. We conclude that quasars are
unlikely to make a significant contribution to reionization.
