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

Linhua Jiang, Yuanhang Ning, Xiaohui Fan, Luis C. Ho, Bin Luo, Feige Wang, Jin Wu, Xue Bing Wu, Jinyi Yang, Zhen Ya Zheng

Research output: Contribution to journalArticlepeer-review

16 Scopus citations


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 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) and galaxies are still under debate. Here we construct the far-ultraviolet luminosity function for type 1 quasars at z ≥ 6 that spans 10 magnitudes (−19 ≤ MUV ≤ −29), conclusively showing that quasars made a negligible contribution to reionization. We mainly search for quasars in the low-luminosity range of MUV > −23 mag that is critical for determination of the total LyC photon production of quasars 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.

Original languageEnglish (US)
Pages (from-to)850-856
Number of pages7
JournalNature Astronomy
Issue number7
StatePublished - Jul 2022

ASJC Scopus subject areas

  • Astronomy and Astrophysics


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