Was the Universe neutral beyond redshift six?

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

Research output: Contribution to conferencePaperpeer-review


We provide measurements of the neutral hydrogen fraction xHI at z ∼ 6, by comparing semi-analytical models of the Lyα forest with observations of high-z quasars and gamma-ray bursts absorption spectra. We analyze the transmitted flux in a sample of 17 QSOs spectra at 5.74 ≤ zem ≤ 6.42 studying 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 our models, we conclude that x HI evolves smoothly from 10-4.4 at z = 5.3 to 10 -4.2 at z = 5.6, with a robust upper limit xHI < 0.36 at z = 6.3. We show the results of the first-ever detected transverse proximity effect in the HI Lyα forest, produced by the HII region of the faint quasar RD J1148+5253 at z = 5.70 intervening along the LOS of SDSS J1148+5251 at z = 6.42. Moreover, we propose a novel method to study cosmic reionization using absorption line spectra of high-redshift GRBs afterglows. We show that the time evolution and the statistics of gaps in the observed spectra represent exquisite tools to discriminate among different reionization models. By applying our methods to GRB 050904 detected at z = 6.29, we show that the observation of this burst provides strong indications of a highly ionized intergalactic medium at z ∼ 6, with an estimated mean neutral hydrogen fraction xHI = 6.4 ± 0.3 × 10-5 along that line of sight.

Original languageEnglish (US)
Number of pages7
StatePublished - 2007
EventA Century of Cosmology: Past, Present and Future - San Servolo, Venice, Italy
Duration: Aug 27 2007Aug 31 2007


OtherA Century of Cosmology: Past, Present and Future
CitySan Servolo, Venice

ASJC Scopus subject areas

  • Astronomy and Astrophysics


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