Volume 447, Number 3, March I 2006
|Page(s)||897 - 903|
|Published online||10 February 2006|
A mean redshift of 2.8 for Swift gamma-ray bursts
Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark e-mail: email@example.com
2 Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK
3 Lund Observatory, Box 43, 221 00 Lund, Sweden
4 Department of Astronomy, Yale University, PO Box 208101, New Haven CT 06520-8101, USA
5 Instituto de Astrofísica de Andalucía (CSIC), Apartado de Correos, 3004, 18080 Granada, Spain
6 Nordic Optical Telescope, Apartado de Correos, 474, 38700 Santa Cruz de la Palma (Tenerife), Spain
7 Joint Astronomy Centre, University Park, 660 North A'ohoku Place, Hilo, HI 96720, USA
8 Instituto de Astrofísica de Canarias, c/. Vía Láctea, s/n, 38200 La Laguna (Tenerife), Spain
9 Observatory, University of Helsinki, PO Box 14, 00014 University of Helsinki, Finland
10 Institute of Theoretical Physics and Astronomy, A. Gostauto St. 12, 01108 Vilnius, Lithuania
11 Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
12 Centro de Astrofísica Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
Accepted: 21 October 2005
The exceptionally high luminosities of gamma-ray bursts (GRBs), gradually emerging as extremely useful probes of star formation, make them promising tools for exploration of the high-redshift Universe. Here we present a carefully selected sample of Swift GRBs, intended to estimate in an unbiased way the GRB mean redshift (), constraints on the fraction of high-redshift bursts and an upper limit on the fraction of heavily obscured afterglows. We find that and that at least 7% of GRBs originate at . In addition, consistent with pre-Swift observations, at most 20% of afterglows can be heavily obscured. The redshift distribution of the sample is qualitatively consistent with models where the GRB rate is proportional to the star formation rate in the Universe. We also report optical, near-infrared and X-ray observations of the afterglow of GRB 050814, which was seen to exhibit very red optical colours. By modelling its spectral energy distribution we find that . The high mean redshift of GRBs and their wide redshift range clearly demonstrates their suitability as efficient probes of galaxies and the intergalactic medium over a significant fraction of the history of the Universe.
Key words: cosmology: observations / dust, extinction / early Universe / galaxies: high-redshift / gamma rays: bursts
© ESO, 2006
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