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

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 $\ion{Si}{ii}$*, $\ion{O}{i}$*, and $\ion{O}{i}$** lines we infer very high densities and low temperatures in the system. There is evidence for the presence of H2 molecules with log $N({\rm H}_2)\sim17.0$, translating into a molecular fraction of $\log{f}\approx -3.5$ with f=2N(H2)/(2N(H2) + $N(\ion{H}{i})$). 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.