Faint active galactic nuclei supplied 31-75% of hydrogen-ionizing photons at z>5

TL;DR

This study uses recent JWST and ground-based data to construct a unified AGN luminosity function at high redshift, showing that faint active galactic nuclei contributed significantly to hydrogen reionization, nearly doubling previous estimates.

Contribution

It provides a new, comprehensive high-redshift AGN luminosity function separating obscured and unobscured populations, and quantifies their substantial role in cosmic reionization.

Findings

AGN contributed 31-75% of ionizing photons at z>5.

The AGN ionizing photon rate is nearly twice previous estimates.

AGN and galaxies jointly sustained reionization, with AGN being a major contributor.

Abstract

The origin of the ionizing photons that completed hydrogen reionization remains debated. Using recent JWST and ground-based surveys at 4.5 <= z <= 6.5, we construct a unified rest-UV AGN luminosity function that separates unobscured Type I and obscured Type II populations, and show that "little red dots" and X-ray selected sources are magnitude-filtered subsets of Type I with a mixture fraction eta = 0.10 +/- 0.02. We anchor the Lyman-continuum (LyC) escape fraction to outflow incidence and geometric clearing rather than assuming quasar-like values for all classes, and propagate uncertainties through a joint fit. Integrating over -27 < M_UV < -17, AGN inject Ndot_ion,AGN = (3.77 +1.08/-0.95) x 10^51 s^-1 Mpc^-3, nearly twice earlier estimates and comparable to the Ly-alpha inferred requirement at z ~ 6. When combined with the JWST galaxy UV luminosity function and a harder stellar ionizing efficiency of log10(xi_ion) = 25.7, AGN contribute 31-75% of the total ionizing photons for representative galaxy escape fractions f_esc,gal = 0.03-0.20. The resulting hydrogen photoionization rate, Gamma_HI ~ (0.5-2) x 10^-12 s^-1 at z ~ 5-6, lies squarely within the Ly-alpha forest constraints once mean free paths and IGM clumpiness are accounted for, remaining consistent for combined AGN-galaxy models up to f_esc,gal <= 5%. These results suggest that AGN and galaxies jointly sustained the ionizing background during the final stages of reionization, with AGN remaining a major but not exclusive contributor.