Glimpsing through the high-redshift neutral hydrogen fog

S. Gallerani, A. Ferrara, X. Fan, T. Roy Choudhury

Research output: Contribution to journalArticlepeer-review

76 Scopus citations


We analyse the transmitted flux in a sample of 17 QSOs spectra at 5.74 ≤ zem ≤ 6.42 to obtain tighter constraints on the volume-averaged neutral hydrogen fraction, xH i, at z ≈ 6. We study separately the narrow transmission windows (peaks) and the wide dark portions (gaps) in the observed absorption spectra. By comparing the statistics of these spectral features with a semi-analytical model of the Lyα forest, we conclude that xH i evolves smoothly from 10-4.4 at z = 5.3 to 10 -4.2 at z = 5.6, with a robust upper limit xH i < 0.36 at z = 6.3. The frequency and physical sizes of the peaks imply an origin in cosmic underdense regions and/or in H ii regions around faint quasars or galaxies. In one case (the intervening H ii region of the faint quasar RD J1148+5253 at z = 5.70 along the line of sight of SDSS J1148+5251 at z = 6.42) the increase of the peak spectral density is explained by the first-ever detected transverse proximity effect in the H i Lyα forest; this indicates that at least some peaks result from a locally enhanced radiation field. We then obtain a strong lower limit on the foreground QSO lifetime of tQ > 11 Myr. The observed widths of the peaks are found to be systematically larger than the simulated ones. Reasons for such discrepancy might reside either in the photoionization equilibrium assumption or in radiative transfer effects.

Original languageEnglish (US)
Pages (from-to)359-369
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - May 2008


  • Intergalactic medium
  • Large-scale structure of Universe
  • Quasars: absorption lines

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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