Evidence for reionization at z ∼ 6:: Detection of a Gunn-Peterson trough in a z=6.28 quasar

We present moderate-resolution Keck spectroscopy of quasars at z = 5.82, 5.99, and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that the Ly alpha absorption in the spectra of these quasars evolves strongly with redshift. To z similar to 5.7, the Ly alpha absorption evolves as expected from an extrapolation from lower redshifts. However, in the highest-redshift object, SDSSp J103027.10+052455.0 (z = 6.28), the average transmitted flux is 0.0038 +/- 0.0026 times that of the continuum level over 8450 Angstrom < <lambda> < 8710 <Angstrom> (5.95 < z(abs) < 6.16), consistent with zero flux. Thus the flux level drops by a factor of greater than 150 and is consistent with zero flux in the Ly alpha forest region immediately blueward of the Ly alpha emission line, compared with a drop by a factor of similar to 10 at z(abs) similar to 5.3. A similar break is seen at Ly beta; because of the decreased oscillator strength of this transition, this allows us to put a considerably stronger limit, tau (eff) > 20, on the optical depth to Ly alpha absorption at z = 6. This is a clear detection of a complete Gunn-Peterson trough, caused by neutral hydrogen in the intergalactic medium. Even a small neutral hydrogen fraction in the intergalactic medium would result in an undetectable flux in the Ly alpha forest region. Therefore, the existence of the Gunn-Peterson trough by itself does not indicate that the quasar is observed prior to the reionization epoch. However, the fast evolution of the mean absorption in these high-redshift quasars suggests that the mean ionizing background along the line of sight to this quasar has declined significantly from z similar to 5 to 6, and the universe is approaching the reionization epoch at z similar to 6.