The fraction of ionized hydrogen left over from the Big Bang provides evidence for the time of formation of the first stars and quasar black holes in the early Universe; such objects provide the high-energy photons necessary to ionize hydrogen. Spectra of the two most distant known quasars1 show nearly complete absorption of photons with wavelengths shorter than the Lyman α transition of neutral hydrogen, indicating that hydrogen in the intergalactic medium (IGM) had not been completely ionized at a redshift of z ≈ 6.3, about one billion years after the Big Bang. Here we show that the IGM surrounding these quasars had a neutral hydrogen fraction of tens of per cent before the quasar activity started, much higher than the previous lower limits1,2 of ∼0.1 per cent. Our results, when combined with the recent inference of a large cumulative optical depth to electron scattering after cosmological recombination3 therefore suggest the presence of a second peak in the mean ionization history of the Universe.
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This work was supported in part by grants from ARC, NSF and NASA.
The authors declare that they have no competing financial interests.
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Wyithe, J., Loeb, A. A large neutral fraction of cosmic hydrogen a billion years after the Big Bang. Nature 427, 815–817 (2004). https://doi.org/10.1038/nature02336
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