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GRB 050814 at z = 5.3 and the Redshift Distribution of Swift GRBs
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GRB 050814 at z = 5.3 and the Redshift
Distribution of Swift GRBs
P. Jakobsson
?
, A. Levan
?
, J. P. U. Fynbo
?
, R. Priddey
?
, J. Hjorth
?
,
N. Tanvir
?
, D. Watson
?
, B. L. Jensen
?
, J. Sollerman
?
, P. Natarajan
??
,
J. Gorosabel
?
, J. M. Castro Cerón
?
and K. Pedersen
?
?
Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30,
2100 Copenhagen, Denmark
?
Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Herts,
AL10 9AB, UK
??
Department of Astronomy, Yale University, PO Box 208101, New Haven CT 06520-8101, USA
?
Instituto de Astrofísica de Andalucía (CSIC), Apartado de Correos, 3004, E-18080 Granada,
Spain
Abstract. We 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 z = 5.3± 0.3. We next present a carefully selected sample of 19 Swift GRBs, intended to
estimate in an unbiased way the GRB redshift distribution, including the mean redshift (zmean) as
well as constraints on the fraction of high-redshift bursts. We find that zmean = 2.7 and that at least
5% of the GRBs originate at z 5. 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. The high
mean redshift of this GRB sample and the wide redshift range clearly demonstrates the suitability
of GRBs as efficient probes of galaxies and the intergalactic medium over a significant fraction of
the history of the Universe.
Keywords: dust, extinction – early Universe – galaxies: high redshift – gamma rays: bursts
PACS: 95.85.Kr – 95.85.Nv – 98.70.Rz
INTRODUCTION
The immense luminosities of the gamma-ray bursts (GRBs), coupled with their origin
in the core collapse of massive stars [1, 2] and their γ-ray penetration through d
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