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 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.
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Data availability
All imaging data used in this paper are publicly available and the details are presented in Table 1 and Methods. Source data are provided with this paper.
Code availability
Data were reduced using publicly available data reduction pipelines.
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Acknowledgements
L.J., Y.N., L.H., J.W. and X.W. acknowledge support from the National Science Foundation of China (grant nos. 11721303, 11890693 and 12022303) and the China Manned Space Project (grant nos. CMS-CSST-2021-A04, CMS-CSST-2021-A05 and CMS-CSST-2021-A06). This Article used observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA).
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L.J. designed the program, analyzed the data, and prepared the manuscript. Y.N. and J.W. performed the image simulations. X.F. and L.H. helped to prepare the manuscript. B.L. and Z.Z. helped with the calculation of the QLF. F.W., X.W and J.Y. helped with the quasar selection. All authors helped with the scientific interpretations and commented on the manuscript.
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Extended data
Extended Data Fig. 1 The z–J versus i–z color-color diagram of high-redshift quasars for different filter sets.
The red, green, and blue circles show the median tracks of the quasar colors calculated for the GOODS, COSMOS, and EGS fields, respectively. The starting redshift is 5.8 and the step size is 0.1. The ending redshifts are different for different fields. The filled circles represent the redshift ranges that we used for target selection.
Extended Data Fig. 2 Color selection completeness maps.
The contours are selection probabilities from 0.8 to 0.2 with an interval of 0.2 for four fields.
Source data
Source Data Fig. 1
Source data for Fig. 1.
Source Data Fig. 2
Source data for Fig. 2.
Source Data Fig. 3
Source data for Fig. 3.
Source Data Extended Data Fig. 1
Source data for Extended Data Fig. 1.
Source Data Extended Data Fig. 2
Source data for Extended Data Fig. 2.
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Jiang, L., Ning, Y., Fan, X. et al. Definitive upper bound on the negligible contribution of quasars to cosmic reionization. Nat Astron 6, 850–856 (2022). https://doi.org/10.1038/s41550-022-01708-w
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DOI: https://doi.org/10.1038/s41550-022-01708-w
