Definitive upper bound on the negligible contribution of quasars to cosmic reionization

TL;DR

This study constructs a comprehensive UV luminosity function for quasars at redshifts 6 and above, conclusively showing their negligible role in cosmic reionization, thereby emphasizing the dominance of star-forming galaxies as ionizing sources.

Contribution

The paper provides the first extensive UV luminosity function for low-luminosity quasars at z >= 6, demonstrating their minimal contribution to reionization.

Findings

Quasars contribute less than 7% of ionizing photons at z=6-6.6.

Galaxies are the primary sources of hydrogen reionization.

Quasars' role in reionization is negligible according to the data.

Abstract

Cosmic (hydrogen) reionization marks one of the major phase transitions of the universe at redshift z >= 6. During this epoch, hydrogen atoms in the intergalactic medium (IGM) were ionized by Lyman continuum (LyC) photons. However, it remains challenging to identify the major sources of the LyC photons responsible for reionization. In particular, individual contributions of quasars (or active galactic nuclei, AGNs) and galaxies are still under debate. Here we construct the far-ultraviolet (far-UV) luminosity function for type 1 quasars at z >= 6 that spans 10 magnitudes (-19 < M_UV < -29), conclusively showing that quasars made a negligible contribution to reionization. We mainly search for quasars in the low-luminosity range of M_UV > -23 mag that is critical to determine quasars' total LyC photon production but has been barely explored previously. We find that the quasar population can only provide less than 7% (95% confidence level) of the total photons needed to keep the universe ionized at z = 6.0 - 6.6. Our result suggests that galaxies, presumably low-luminosity star-forming systems, are the major sources of hydrogen reionization.