A large neutral fraction of cosmic hydrogen a billion years after the Big Bang

Abstract

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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Figure 1: Predicted probability for observing different radii of the ionized region around the quasar SDSS J1030 + 0524.
Figure 2: Likelihood for the inferred neutral fraction of the IGM, assuming different quasar lifetimes.

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Acknowledgements

This work was supported in part by grants from ARC, NSF and NASA.

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Correspondence to Abraham Loeb.

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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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