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

J P U Fynbo,E Rol,K Wiersema,K Pedersen,D H Hartmann,P M Vreeswijk,G Björnsson,R A M J Wijers,M Limousin,S T Holland,G Nelemans,P Møller,B L Jensen,R L C Starling,C Ledoux,J M Castro Cerón,A J Levan,J Gorosabel,C C Thöne,P A Curran ,Jyri Näränen ,C Kouveliotou,R Priddey ,Derrick G Watson ,P Jakobsson,J Sollerman,J Hjorth,N R Tanvir

doi:10.1051/0004-6361:20065056

Abstract

Aim: 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. Method: 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-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 and low temperatures in the system. There is evidence for the presence of H$_2$ molecules with logN(H_2) ~ 17.0, translating into a molecular fraction of logf \~ -3.5 with f=2N(H_2)/(2N(H_2)+ N(Hi)). Even if GRBs are only formed by single massive stars with metallicities below ~0.3Z(solar), they could still be fairly unbiased tracers of the bulk of the star formation at z>2. Hence, metallicities as derived for GRB060206 here for a complete sample of GRB afterglows will directly show the distribution of metallicities for representative star-forming galaxies at these redshifts.

Highlights

In the analysis presented here we focus on the metallicity of the gamma-ray bursts (GRBs) absorption system
The profile of the GRB absorption systems consists of at least four components spread over ∼500 km s−1 in velocity space
Despite its high redshift, the GRB 060206 system has one of the highest metallicities measured for a GRB absorption system and one of the highest metallicities measured from QSO absorption lines at z > 4

Summary

Introduction

Two consistent features are seen at the location of the W1−0 R(0), W1−0 R(1) and W1−0 Q(1) lines at zabs = 4.04793 (bottom panel in Fig. 1), with column densities of log N(H2) ∼ 17.0 for the J = 1 rotational level, and log N(H2) < 16.7 for J = 0. Lower panel: the likely H2 lines at the GRB redshift in the MR spectrum.

Results

Conclusion