The Contribution of AGN Accretion Disks to Hydrogen Reionization
  We examine the contribution of high-redshift (z>6) active galactic nuclei
(AGNs) to cosmic hydrogen reionization, by tracing the growth and ionizing
output of the first generation of supermassive black holes (SMBHs). Our
calculations are anchored to the observed population of z~6 quasars, and trace
back the evolving spectral energy distributions (SEDs) of the accretion flows
that power these early AGNs and consider a variety of growth histories,
including super-Eddington accretion. Compared to a fixed-shape SED, the
evolving thin disks can produce ionizing radiation that is higher by up to
~80%. Across a variety of SMBH growth scenarios, the contribution of AGNs to
reionization is limited to late epochs (z<7), and remains sub-dominant compared
to star-forming galaxies. This conclusion holds irrespective of the (still
unknown) space density of low-luminosity z=6 AGNs, and for growth scenarios
that allow super-Eddington accretion. The contribution of AGNs to reionization
can extend to earlier epochs (z>8) in scenarios with relative slow SMBH mass
growth, i.e., for low accretion rates and/or high spins. We finally demonstrate
that our framework can reproduce the observed quasar proximity zone sizes, and
that compact proximity zones around z=6 quasars can be explained by the late
onset of super-Eddington accretion.
