Document Type

Article

Publication Date

6-2006

Publication Title

Astronomy & Astrophysics

Volume

451

Issue

3

Publisher

EDP Sciences

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 ±0.00020. Based on the measurement of the neutral hydrogen column density from the damped Lyman-αline and the metal content from weak, unsaturated S ii lines we derive a metallicity of [S/H] =−0.84 ±0.10. This is one of the highest metallicities measured from absorption lines at z ∼4. From the very high column densities for the forbidden Siii*, O i*, and O i** lines we infer very high densities and low temperatures in the system. There is evidence for the presence of H2 molecules with log N(H2) ∼17.0, translating into a molecular fraction of log f ≈−3.5 with f =2N(H2)/(2N(H2) +N(H i)). Even if GRBs are only formed by single massive stars with metallicities below ∼0.3 Z, 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.

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This manuscript has been published in the Astronomy & Astrophysics Journal. Please find the published version here (note that a subscription is necessary to access this version):

http://www.aanda.org/articles/aa/abs/2006/21/aa5056-06/aa5056-06.html

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