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

Robert H. Becker, Xiaohui Fan, Richard L. White, Michael A. Strauss, Vijay K. Narayanan, Robert H. Lupton, James E. Gunn, James Annis, Neta A. Bahcall, J. Brinkmann, A. J. Connolly, István Csabai, Paul C. Czarapata, Mamoru Doi, Timothy M. Heckman, G. S. Hennessy, Željko Ivezić, G. R. Knapp, Don Q. Lamb, Timothy A. MckayJeffrey A. Munn, Thomas Nash, Robert Nichol, Jeffrey R. Pier, Gordon T. Richards, Donald P. Schneider, Chris Stoughton, Alexander S. Szalay, Aniruddha R. Thakar, D. G. York

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801 Scopus citations


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 Lyα absorption in the spectra of these quasars evolves strongly with redshift. To z ∼ 5.7, the Lyα 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 Å < λ < 8710 Å (5.95 < zabs < 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 Lyα forest region immediately blueward of the Lyα emission line, compared with a drop by a factor of ∼10 at zabs ∼ 5.3. A similar break is seen at Lyβ; because of the decreased oscillator strength of this transition, this allows us to put a considerably stronger limit, τeff > 20, on the optical depth to Lyα 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 Lyα 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 ∼ 5 to 6, and the universe is approaching the reionization epoch at z ∼ 6.

Original languageEnglish (US)
Pages (from-to)2850-2857
Number of pages8
JournalAstronomical Journal
Issue number6
StatePublished - Dec 2001

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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