Physical conditions in high-redshift GRB-DLA absorbers observed with VLT/UVES: Implications for molecular hydrogen searches

Abstract

Aims. We aim to understand the nature of the absorbing neutral gas in the galaxies hosting high-redshift long-duration gamma-ray bursts (GRBs) and to determine their physical conditions. Methods.A detailed analysis of high-quality VLT/UVES spectra of the optical afterglow of GRB050730 and other Swift-era bursts is presented. Results.We report the detection of a significant number of previously unidentified allowed transition lines of Fe+, involving the fine structure of the ground term (6D7/2, 6D5/2, 6D3/2, 6D1/2) and that of other excited levels (4F9/2, 4F7/2, 4F5/2, 4F3/2, 4D7/2, 4D5/2), from the zabs = 3.969, log N(H0) = 22.10, damped Lyman-á (DLA) system located in the host galaxy of GRB050730. No molecular hydrogen (H2) is detected down to a molecular fraction of log f < -3) in a sample of seven z > 1.8 GRB host galaxies observed with VLT/UVES. We show that the lack of H2 can be explained by the low metallicities, [X/H]