A Gunn-Peterson test with a QSO at z=6.4
  Understanding the cosmic re-ionization is one of the key goals of the modern
observational cosmology. High redshift QSO spectra can be used as background
light sources to measure absorption by intervening neutral hydrogen. We
investigate neutral hydrogen absorption in a deep, moderate-resolution
Keck/Deimos spectrum of QSO CFHQSJ2329-0301 at z=6.4. This QSO is one of the
highest redshift QSOs presently known at z=6.4 but is 2.5 mag fainter than a
previously well-studied QSO SDSSJ1148+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 optical depth,
tau_eff>4.9. These results are consistent with strong evolution of the optical
depth at z>5.7.
