Title: Constraining the contribution of active galactic nuclei to reionisation,
Abstract: 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.