Probing Cosmic Chemical Evolution with Gamma-Ray Bursts: GRB060206 at z=4.048

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  • 20 February 2006
Abstract
We present early NOT/ALFOSC and WHT/ISIS optical spectroscopy of the afterglow of the gamma-ray burst GRB060206. We measure a redshift of z=4.0480+/-0.0002. Based on the measurement of the neutral hydrogen column density from the damped Lyman-alpha line and the metal content from weak, unsaturated SII 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*, OI*, and OI** lines we infer very high densities in the system, significantly larger than 10^4 cm^-3. There is also tentative evidence for the presence of H_2 molecules corresponding to log N(H_2)~15.2 for both J=0 and J=1, translating to a molecular fraction of log(f)~-5 with f=2N(H_2)/(2N(H_2)+N(HI)). We also discuss how GRBs can help resolve the so-called missing metals problem. Although GRBs may only be formed by massive stars with metallicities below ~0.25 Z_sun, we argue that they will still be fairly unbiased tracers of star formation at z>2. Hence, metallicities derived as in this Letter for a complete sample of GRB afterglows will directly show the distribution of metallicities for star-forming galaxies at these redshifts. Moreover, the distribution of GRB host galaxy luminosities will be crucial for the determination of the faint end slope of the luminosity function which in turn is important for determining the cosmic star-formation density.

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