On the Contribution of Active Galactic Nuclei to the High-Redshift
  Metagalactic Ionizing Background
  Motivated by the claimed detection of a large population of faint active
galactic nuclei (AGN) at high redshift, recent studies have proposed models in
which AGN contribute significantly to the z > 4 H I ionizing background. In
some models, AGN are even the chief sources of reionization. If correct, these
models would make necessary a complete revision to the standard view that
galaxies dominated the high-redshift ionizing background. It has been suggested
that AGN-dominated models can better account for two recent observations that
appear to be in conflict with the standard view: (1) large opacity variations
in the z ~ 5.5 H I Lyman-alpha forest, and (2) slow evolution in the mean
opacity of the He II Lyman-alpha forest. Large spatial fluctuations in the
ionizing background from the brightness and rarity of AGN may account for the
former, while the earlier onset of He II reionization in these models may
account for the latter. Here we show that models in which AGN emissions source
>~ 50 % of the ionizing background generally provide a better fit to the
observed H I Lyman-alpha forest opacity variations compared to standard
galaxy-dominated models. However, we argue that these AGN-dominated models are
in tension with constraints on the thermal history of the intergalactic medium
(IGM). Under standard assumptions about the spectra of AGN, we show that the
earlier onset of He II reionization heats up the IGM well above recent
temperature measurements. We further argue that the slower evolution of the
mean opacity of the He II Lyman-alpha forest relative to simulations may
reflect deficiencies in current simulations rather than favor AGN-dominated
models as has been suggested.
