Was the Universe neutral beyond redshift six?

TL;DR

This paper measures the neutral hydrogen fraction at high redshift using quasar and gamma-ray burst spectra, revealing a smoothly evolving ionization state and providing new methods to study cosmic reionization.

Contribution

It introduces a novel approach combining spectral feature analysis with models to constrain the hydrogen neutral fraction and studies the first transverse proximity effect at high redshift.

Findings

Neutral hydrogen fraction xHI evolves smoothly from 10^{-4.4} to 10^{-4.2} between z=5.3 and 5.6.

Upper limit of xHI<0.36 at z=6.3 indicating significant ionization.

Detection of the transverse proximity effect from a faint quasar at z=5.70.

Abstract

We provide measurements of the neutral hydrogen fraction xHI at z~6, by comparing semi-analytical models of the Lyalpha 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 xHI 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 Lyalpha 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\pm 0.3\times 10^{-5} along that line of sight.