A photometric redshift of z = 6.39 ± 0.12 for GRB 050904

Long ago and far away The γ-ray burst GRB 050904, detected by the Swift satellite on 4 September last year, is one of the most distant objects ever observed. Its redshift of z = 6.3 equates to an explosion taking place 12.8 billion years ago, when the Universe was a mere 890 million years old. Three groups this week present detailed observations of the γ-ray, X-ray, near-infrared and optical spectra of the afterglow of GRB 050904. The results begin to paint a picture of the conditions prevailing when the parent body exploded and suggest that the γ-ray bursts that we see in the future can be used by cosmologists to probe the early Universe for evidence of star and galaxy formation, nucleosynthesis and reionization. Gamma-ray bursts (GRBs) and their afterglows are the most brilliant transient events in the Universe. Both the bursts themselves and their afterglows have been predicted to be visible out to redshifts of z ≈ 20, and therefore to be powerful probes of the early Universe 1 , 2 . The burst GRB 000131, at z = 4.50, was hitherto the most distant such event identified 3 . Here we report the discovery of the bright near-infrared afterglow of GRB 050904 (ref. 4 ). From our measurements of the near-infrared afterglow, and our failure to detect the optical afterglow, we determine the photometric redshift of the burst to be (refs 5–7 ). Subsequently, it was measured 8 spectroscopically to be z = 6.29 ± 0.01, in agreement with our photometric estimate. These results demonstrate that GRBs can be used to trace the star formation, metallicity, and reionization histories of the early Universe.