Semi-analytic forecasts for JWST -- V. AGN luminosity functions and
  helium reionization at z = 2-7
  Active galactic nuclei (AGN) forming in the early universe are thought to be
the primary source of hard ionizing photons contributing to the reionization of
intergalactic helium. However, the number density and spectral properties of
high-redshift AGN remain largely unconstrained. In this work, we make use of
physically-informed models calibrated with a wide variety of available
observations to provide estimates for the role of AGN throughout the Epoch of
Reionization. We present AGN luminosity functions in various bands between z =
2 to 7 predicted by the well-established Santa Cruz semi-analytic model, which
includes modelling of black hole accretion and AGN feedback. We then combine
the predicted AGN populations with a physical spectral model for
self-consistent estimates of ionizing photon production rates, which depend on
the mass and accretion rate of the accreting supermassive black hole. We then
couple the predicted comoving ionizing emissivity with an analytic model to
compute the subsequent reionization history of intergalactic helium and
hydrogen. This work demonstrates the potential of coupling physically motivated
analytic or semi-analytic techniques to capture multi-scale physical processes
across a vast range of scales (here, from AGN accretion disks to cosmological
scales). Our physical model predicts an intrinsic ionizing photon budget well
above many of the estimates in the literature, meaning that helium reionization
can comfortably be accomplished even with a relatively low escape fraction. We
also make predictions for the AGN populations that are expected to be detected
in future \emph{James Webb Space Telescope} surveys.
