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

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