HI column densities of z > 2 Swift gamma-ray bursts

Context. Before the launch of the Swift satellite, the majority of the gamma-ray burst (GRB) afterglows for which Lya was redshifted into the observable spectrum showed evidence for a damped Lya absorber. This small sample indicated that GRBs explode either in galaxies, or regions within them, having high neutral hydrogen column densities. Aims. To increase the spectroscopic sample of GRBs with z > 2 and hence establish the N(H I) distribution along GRB lines-of-sight. Methods. We have obtained six z > 2 GRB afterglow spectra and fitted the Lya absorption line in each case to determine N(H I). This has been complemented with 12 other Swift N(H I) values from the literature. Results. We show that the peak of the GRB N(H I) distribution is qualitatively consistent with a model where GRBs originate in Galactic-like molecular clouds. However, a systematic difference, in particular an excess of low column-density systems compared to the predictions, indicates that selection effects and conditions within the cloud (e.g. strong ionization) influence the observed N(H I) range. We also report the discovery of Lya emission from the GRB 060714 host, corresponding to a star-formation rate of approximately 0.8 M(circle dot) yr(-1). Finally, we present accurate redshifts of the six bursts: z = 3.240 +/- 0.001 (GRB 050319), z = 2.198 +/- 0.002 (GRB 050922C), z = 3.221 +/- 0.001 (GRB 060526), z = 3.425 +/- 0.002 (GRB 060707), z = 2.711 +/- 0.001 (GRB 060714) and z = 3.686 +/- 0.002 (GRB 060906).