Abstract
Understanding the cosmic reionization is one of the key goals of the modern observational cosmology. High-redshift quasi-stellar object (QSO) spectra can be used as background light sources to measure the absorption by the intervening neutral hydrogen. We investigate the neutral hydrogen absorption in a deep, moderate-resolution Keck/Deimos spectrum of the QSO CFHQS J2329-0301 atz= 6.4. This QSO is one of the highest redshift QSOs presently known atz= 6.4 but is 2.5mag fainter than a previously well-studied QSO SDSS J1148+5251 atz= 6.4. Therefore, it has a smaller Stromgren sphere and allows us to probe the highest redshift hydrogen absorption to date. The average transmitted flux at 5.915 <z <sub>abs</sub> < 6.365 (200 comoving Mpc) is consistent with zero in Lyα, Lyβ and Lyγ absorption measurements. This corresponds to the lower limit of the optical depth,τ <sup>α</sup> <sub>eff</sub> > 4.9. These results are consistent with the strong evolution of the optical depth atz> 5.7. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.