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

Mainak Singha, Sangeeta Malhotra, James Ely Rhoads

First listed 2025-10-30 | Last updated 2025-10-30

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.

Short digest

Using 4.5≤z≤6.5 samples from JWST and ground-based surveys, the authors build a unified rest-UV AGN luminosity function with two physical populations—Type I fit by a Schechter form and Type II by a double power law—showing that little red dots and X-ray–selected sources are magnitude-filtered Type I with η=0.10±0.02. They tie the Lyman-continuum escape fraction to outflow-driven geometric clearing, yielding luminosity-averaged fesc≈0.64 for faint Type I, 0.25 for Type II, and ≈0.91 for bright quasars. Integrating −27<MUV<−17 gives Ndot_ion,AGN=(3.77+1.08/−0.95)×10^51 s^−1 Mpc^−3, so AGN provide 31–75% of the ionizing photons at z≈5–6 when combined with ξion=25.7 and galaxy fesc=0.03–0.20. The implied ΓHI≈(0.5–2)×10^−12 s^−1 matches Lyα-forest constraints, pointing to AGN as a major—but not sole—driver of the late reionization background.

Key figures to inspect

• Rest-UV AGN luminosity function decomposed into Type I (Schechter) and Type II (double power law), with LRD/XRB overplotted as η=0.10±0.02 scaled subsets—inspect residuals and ΔAIC/ΔBIC to see why only two populations are required.
• Class-dependent LyC escape fraction vs MUV from the outflow-clearing model (Rout/Rreq, Φgeom): read off fesc≈0.64 (faint Type I), 0.25 (Type II), 0.91 (bright quasars), 0.08 (X-ray weak) and how fesc increases with luminosity.
• Ionizing photon production integrated over −27<MUV<−17: Ndot_ion,AGN with uncertainty band; compare to earlier estimates and to the Lyα-inferred requirement at z∼6.
• Predicted ΓHI at z≈5–6 with bands for different galaxy fesc assumptions, plotted against Lyα-forest measurements after mean free path and IGM clumpiness corrections—check where models remain consistent.
• Link between Lbol/MBH/λEdd and outflow incidence (Pwind) showing where Rout>Rreq; see how this threshold sets Φgeom and the fraction of cleared vs screened sightlines across classes.