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


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.


  1. 1

    White, R. L., Becker, R. H., Fan, X. & Strauss, M. A. Probing the ionization state of the universe at z > 6. Astron. J. 126, 1–14 (2003)

    ADS  CAS  Article  Google Scholar 

  2. 2

    Fan, X. et al. Evolution of the ionizing background and the epoch of reionization from the spectra of z 6 quasars. Astron. J. 123, 1247–1257 (2002)

    ADS  Article  Google Scholar 

  3. 3

    Kogut, A. et al. First-year Wilkinson microwave anisotropy probe (WMAP) observations: temperature-polarization correlation. Astron. J. Suppl. 148, 161–173 (2003)

    ADS  Article  Google Scholar 

  4. 4

    Hu, E. M. et al. A redshift z = 6.56 galaxy behind the cluster Abel 370. Astrophys. J. 568, L75–L79 (2002)

    ADS  CAS  Article  Google Scholar 

  5. 5

    Haiman, Z. The detectability of high-redshift Lyα emission lines prior to the reionization of the universe. Astrophys. J. 576, L1–L4 (2002)

    ADS  Article  Google Scholar 

  6. 6

    Willott, C. J., McLure, R. J. & Jarvis, M. J. A 3 × 109 M black hole in the quasar SDSS J1148 + 5251 at z = 6.41. Astrophys. J. 587, L15–L18 (2003)

    ADS  Article  Google Scholar 

  7. 7

    Walter, F. et al. Molecular gas in the host galaxy of a quasar at redshift z = 6.42. Nature 424, 406–408 (2003)

    ADS  CAS  Article  Google Scholar 

  8. 8

    Richards, G. T. et al. Broad emission-line shifts in quasars: an orientation measure for radio-quiet quasars? Astron. J. Suppl. 124, 1–17 (2002)

    ADS  CAS  Article  Google Scholar 

  9. 9

    Barkana, R. & Loeb, A. In the beginning: the first sources of light and the reionization of the universe. Phys. Rep. 349, 125–238 (2001)

    ADS  CAS  Article  Google Scholar 

  10. 10

    Gunn, J. E. & Peterson, B. A. On the density of neutral hydrogen in intergalactic space. Astrophys. J. 142, 1633–1641 (1965)

    ADS  CAS  Article  Google Scholar 

  11. 11

    Pentericci, L. et al. VLT optical and near-infrared observations of the z = 6.28 quasar SDSS J1030 + 0524. Astron. J. 123, 2151–2158 (2002)

    ADS  CAS  Article  Google Scholar 

  12. 12

    Martini, P. QSO lifetimes. Preprint at 〈http://arXiv.org/astro-ph/0304009〉 (2003).

  13. 13

    Miralda-Escude, J., Haehnelt, M. & Rees, M. Reionization of the inhomogeneous universe. Astrophys. J. 530, 1–16 (2000)

    ADS  CAS  Article  Google Scholar 

  14. 14

    Barkana, R. & Loeb, A. GRBs versus Quasars: Lyman-α signatures of reionization verses cosmological infall. Astrophys. J. (in the press); preprint at 〈http://arXiv.org/astro-ph/0305470〉 (2003)

  15. 15

    Shapiro, P. R. & Giroux, M. L. Cosmological, H II regions and the photoionization of the intergalactic medium. Astrophys. J. 321, L107–L112 (1987)

    ADS  CAS  Article  Google Scholar 

  16. 16

    Cen, R. & Haiman, Z. Quasar Stromgren spheres before cosmological reionization. Astrophys. J. 542, L74–L78 (2000)

    ADS  Article  Google Scholar 

  17. 17

    Madau, P. & Rees, M. J. The earliest luminous sources and the damping wing of the Gunn-Peterson trough. Astrophys. J. 542, L69–L73 (2000)

    ADS  CAS  Article  Google Scholar 

  18. 18

    Telfer, R. C., Zheng, W., Kriss, G. A. & Davidsen, A. F. The rest-frame extreme-ultraviolet spectral properties of quasi-stellar objects. Astron. J. 565, 773–785 (2002)

    ADS  CAS  Article  Google Scholar 

  19. 19

    Elvis, M. et al. Atlas of quasar energy distributions. Astron. J. Suppl. 95, 1–68 (1994)

    ADS  Article  Google Scholar 

  20. 20

    Shields, G. A. et al. The black hole-bulge relationship in quasars. Astrophys. J. 583, 124–133 (2003)

    ADS  Article  Google Scholar 

  21. 21

    Ferrarese, L. Beyond the Bulge: a fundamental relationship between supermassive black holes and dark matter halos. Astrophys. J. 587, 90–97 (2001)

    Google Scholar 

  22. 22

    Volonteri, M., Haardt, F. & Madau, P. The assembly and merging history of supermassive black holes in hierarchical models of galaxy formation. Astrophys. J. 582, 559–573 (2003)

    ADS  Article  Google Scholar 

  23. 23

    Yu, Q. & Tremaine, S. Observational constraints on growth of massive black holes. Mon. Not. R. Astron. Soc. 335, 965–976 (2002)

    ADS  Article  Google Scholar 

  24. 24

    Martini, P. & Weinberg, D. H. Quasar clustering and the lifetime of quasars. Astrophys. J. 547, 12–26 (2001)

    ADS  Article  Google Scholar 

  25. 25

    Jakobsen, P., Jansen, R. A., Wagner, S. & Reimers, D. Caught in the act: a helium-reionizing quasar near the line of sight to Q0302003. Astron. Astrophys. 397, 891–898 (2003)

    ADS  CAS  Article  Google Scholar 

  26. 26

    Martini, P. & Schneider, D .P. Preprint at 〈http://arXiv.org/astro-ph/0309650〉 (2003).

  27. 27

    Wyithe, J. S. B. & Loeb, A. Reionization of hydrogen and helium by early stars and quasars. Astrophys. J. 586, 693–708 (2003)

    ADS  CAS  Article  Google Scholar 

  28. 28

    Cen, R. The implications of Wilkinson microwave anisotropy probe observations for population III star formation processes. Astrophys. J. 591, L5–L8 (2003)

    ADS  Article  Google Scholar 

  29. 29

    Miralda-Escude, J. Reionization of the intergalactic medium and the damping wing of the Gunn-Peterson trough. Astrophys. J. 501, 15–22 (1998)

    ADS  CAS  Article  Google Scholar 

  30. 30

    Spergel, D. N. et al. First-year Wilkinson microwave anisotropy probe (WMAP) observations: determination of cosmological parameters. Astron. J. Suppl. 148, 175–194 (2003)

    ADS  Article  Google Scholar 

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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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