A Gunn-Peterson test with a QSO at z=6.4

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 at z = 6.4. This QSO is one of the highest redshift QSOs presently known at z = 6.4 but is 2.5mag fainter than a previously well-studied QSO SDSS J1148+5251 at z = 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(abs) < 6.365 (200 comoving Mpc) is consistent with zero in Ly alpha, Ly beta and Ly gamma absorption measurements. This corresponds to the lower limit of the optical depth, tau(alpha)(eff) > 4.9. These results are consistent with the strong evolution of the optical depth at z > 5.7.