Constraining the contribution of active galactic nuclei to reionisation
  Recent results have suggested that active galactic nuclei (AGN) could provide
enough photons to reionise the Universe. We assess the viability of this
scenario using a semi-numerical framework for modeling reionisation, to which
we add a quasar contribution by constructing a Quasar Halo Occupation
Distribution (QHOD) based on Giallongo et al. observations. Assuming a constant
QHOD, we find that an AGN-only model cannot simultaneously match observations
of the optical depth $\tau_e$, neutral fraction, and ionising emissivity. Such
a model predicts $\tau_e$ too low by $\sim 2\sigma$ relative to Planck
constraints, and reionises the Universe at $z\lesssim 5$. Arbitrarily
increasing the AGN emissivity to match these results yields a strong mismatch
with the observed ionising emissivity at $z\sim 5$. If we instead assume a
redshift-independent AGN luminosity function yielding an emissivity evolution
like that assumed in Madau & Haardt model, then we can match $\tau_e$ albeit
with late reionisation, however such evolution is inconsistent with
observations at $z\sim 4-6$ and poorly motivated physically. These results
arise because AGN are more biased towards massive halos than typical reionising
galaxies, resulting in stronger clustering and later formation times.
AGN-dominated models produce larger ionising bubbles that are reflected in
$\sim\times 2$ more 21cm power on all scales. A model with equal parts galaxies
and AGN contribution is still (barely) consistent with observations, but could
be distinguished using next-generation 21cm experiments HERA and SKA-low. We
conclude that, even with recent claims of more faint AGN than previously
thought, AGN are highly unlikely to dominate the ionising photon budget for
reionisation.
