Evidence for Reionization at [ITAL][CLC]z[/CLC][/ITAL] ∼ 6: Detection of a Gunn-Peterson Trough in a [ITAL][CLC]z[/CLC][/ITAL] = 6.28 Quasar

We present moderate resolution Keck spectroscopy of quasars at z=5.82, 5.99and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that theLy Alpha absorption in the spectra of these quasars evolves strongly withredshift. To z~5.7, the Ly Alpha absorption evolves as expected from anextrapolation from lower redshifts. However, in the highest redshift object,SDSSp J103027.10+052455.0 (z=6.28), the average transmitted flux is0.0038+-0.0026 times that of the continuum level over 8450 A < lambda < 8710 A(5.95150, and is consistent with zero flux in the Ly Alpha forest regionimmediately blueward of the Ly Alpha emission line, compared with a drop by afactor of ~10 at z(abs)~5.3. A similar break is seen at Ly Beta; because of thedecreased oscillator strength of this transition, this allows us to put aconsiderably stronger limit, tau(eff) > 20, on the optical depth to Ly Alphaabsorption at z=6. This is a clear detection of a complete Gunn-Peterson trough, caused byneutral hydrogen in the intergalactic medium. Even a small neutral hydrogenfraction in the intergalactic medium would result in an undetectable flux inthe Ly Alpha forest region. Therefore, the existence of the Gunn-Petersontrough by itself does not indicate that the quasar is observed prior to thereionization epoch. However, the fast evolution of the mean absorption in thesehigh-redshift quasars suggests that the mean ionizing background along the lineof sight to this quasar has declined significantly from z~5 to 6, and theuniverse is approaching the reionization epoch at z~6.Comment: Revised version (2001 Sep 4) accepted by the Astronomical Journal (minor changes