H I 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 Ly$\alpha$ was redshifted into the observable spectrum showed evidence for a damped Ly$\alpha$ 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 $N(\ion{H}{i})$ distribution along GRB lines-of-sight. Methods.We have obtained six z > 2 GRB afterglow spectra and fitted the Ly$\alpha$ absorption line in each case to determine $N(\ion{H}{i})$. This has been complemented with 12 other Swift $N(\ion{H}{i})$ values from the literature. Results.We show that the peak of the GRB $N(\ion{H}{i})$ 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 $N(\ion{H}{i})$ range. We also report the discovery of Ly$\alpha$ emission from the GRB 060714 host, corresponding to a star-formation rate of approximately 0.8 $M_{\odot}\,{\rm yr}^{-1}$. Finally, we present accurate redshifts of the six bursts: $z = 3.240 \pm 0.001$ (GRB 050319), $z = 2.198 \pm 0.002$ (GRB 050922C), $z = 3.221 \pm 0.001$ (GRB 060526), $z = 3.425 \pm 0.002$ (GRB 060707), $z = 2.711 \pm 0.001$ (GRB 060714) and $z = 3.686 \pm 0.002$ (GRB 060906).