Astronomy & Astrophysics

Free access article

A&A 451, L47-L50 (2006)
DOI: 10.1051/0004-6361:20065056

Letter

Probing cosmic chemical evolution with gamma-ray bursts: GRB 060206 at z = 4.048

J. P. U. Fynbo¹, R. L. C. Starling², C. Ledoux³, K. Wiersema², C. C. Thöne¹, J. Sollerman¹, P. Jakobsson¹, J. Hjorth¹, D. Watson¹, P. M. Vreeswijk^{4, 3}, P. Møller⁵, E. Rol⁶, J. Gorosabel⁷, J. Näränen⁸, R. A. M. J. Wijers², G. Björnsson⁹, J. M. Castro Cerón¹, P. Curran², D. H. Hartmann¹⁰, S. T. Holland¹¹, B. L. Jensen¹, A. J. Levan¹², M. Limousin¹, C. Kouveliotou¹³, G. Nelemans¹⁴, K. Pedersen¹, R. S. Priddey¹² and N. R. Tanvir¹²

¹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
e-mail: jfynbo@astro.ku.dk
² Astronomical Institute `Anton Pannekoek', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
⁴ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁵ European Southern Observatory, Karl-Schwarzschild-strasse 2, 85748 Garching bei München, Germany
⁶ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁷ Instituto de Astrofísica de Andalucía (CSIC), Apartado de Correos 3004, 18080 Granada, Spain
⁸ Observatory, University of Helsinki, PO Box 14, 00014 Helsinki, Finland
⁹ Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland
¹⁰ Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29634-0978, USA
¹¹ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹² Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UK
¹³ NASA Marshall Space Flight Center, NSSTC, XD-12, 320 Sparkman Drive, Huntsville, AL 35805, USA
¹⁴ Department of Astrophysics, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands

(Received 20 February 2006 / Accepted 2 April 2006 )

Abstract
Aims.We present early optical spectroscopy of the afterglow of the gamma-ray burst GRB 060206 with the aim of determining the metallicity of the GRB absorber and the physical conditions in the circumburst medium. We also discuss how GRBs may be important complementary probes of cosmic chemical evolution.
Methods.Absorption line study of the GRB afterglow spectrum.
Results.We determine the redshift of the GRB to be $z=4.04795\pm0.00020$ . Based on the measurement of the neutral hydrogen column density from the damped Lyman- $\alpha$ line and the metal content from weak, unsaturated $\ion{S}{ii}$ lines we derive a metallicity of $\rm [S/H]=-0.84\pm0.10$ . This is one of the highest metallicities measured from absorption lines at $z\sim4$ . From the very high column densities for the forbidden $\ion{ii}$ *, $\ion$ *, and $\ion$ ** lines we infer very high densities and low temperatures in the system. There is evidence for the presence of H₂ molecules with log $N({\rm H}_2)\sim17.0$ , translating into a molecular fraction of $\log{f}\approx -3.5$ with f=2N(H₂)/(2N(H₂) + $N(\ion)$ ). Even if GRBs are only formed by single massive stars with metallicities below ${\sim}0.3~Z_{\odot}$ , they could still be fairly unbiased tracers of the bulk of the star formation at z>2. Hence, metallicities as derived for GRB 060206 here for a complete sample of GRB afterglows will directly show the distribution of metallicities for representative star-forming galaxies at these redshifts.

Key words: gamma rays: bursts -- galaxies: high-redshift -- galaxies: abundances -- cosmology: observations

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