The prompt -ray emission from -ray bursts (GRBs) should be detectable out to distances of z > 10 (ref. 1), and should therefore provide an excellent probe of the evolution of cosmic star formation, reionization of the intergalactic medium, and the metal enrichment history of the Universe^{1, 2, 3, 4}. Hitherto, the highest measured redshift for a GRB has been z = 4.50 (ref. 5). Here we report the optical spectrum of the afterglow of GRB 050904 obtained 3.4 days after the burst; the spectrum shows a clear continuum at the long-wavelength end of the spectrum with a sharp cut-off at around 9,000 Å due to Lyman absorption at z 6.3 (with a damping wing). A system of absorption lines of heavy elements at z = 6.295 0.002 was also detected, yielding the precise measurement of the redshift. The Si ii fine-structure lines suggest a dense, metal-enriched environment around the progenitor of the GRB.

1. Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
2. National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
3. Subaru Telescope, National Astronomical Observatory of Japan, 650 North A'ohoku Place, Hilo, Hawaii 96720, USA
4. Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto, 606-8502, Japan
5. University of California at Berkeley, Space Sciences Laboratory, 7 Gauss Way, Berkeley, California 94720-7450, USA
6. Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
7. Institute of Astronomy, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
8. Department of Astronomy, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan
9. RIKEN (Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
10. Department of Physics, Aoyama Gakuin University, Sagamihara, Kanagawa 229-8558, Japan

Correspondence to: N. Kawai¹ Correspondence and requests for materials should be addressed to N.K. (Email: nkawai@phys.titech.ac.jp).

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