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 (AGNs) at high redshift, recent studies have proposed models in which AGNs contribute significantly to the z > 4 H i ionizing background. In some models, AGNs are even the chief sources of reionization. If proved true, 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 Ly α forest, and (2) slow evolution in the mean opacity of the He ii Ly α forest. Large spatial fluctuations in the ionizing background from the brightness and rarity of AGNs 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 per cent of the ionizing background generally provide a better fit to the observed H i Ly α 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 AGNs, 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 Ly α forest relative to simulations may reflect deficiencies in current simulations rather than favour AGN-dominated models as has been suggested.