Heavy Element Abundances and Dust Depletions in the Host Galaxies of Three Gamma-Ray Bursts

We have derived the column densities of heavy elements in three gamma-ray burst (GRB) optical transients, associated with the circumburst or interstellar medium (ISM) of the host galaxy. In comparison with the same elements observed in damped Lyman-alpha (DLA) systems along QSO sight lines, we find evidence for much higher column densities of ZnII. The gap between the QSO-DLA and GRB-DLA distributions is smoothly bridged by observations of the interstellar absorption in the Milky Way and the Magellanic Clouds. Very small [Fe/Zn], [Si/Zn], and [Cr/Zn] values in GRB-DLAs indicate large dust depletions. Once the dust-to-metals ratios are determined, we find an optical extinction A_V ~ 1 mags, to be compared with typical A_V < 0.1 in most QSO-DLAs. Our inference of high dust content appears to be in contradiction with the typical low reddening previously found in GRBs. One possible way to reconcile is a dust grain-size distribution biased towards big grains, which would give a grey extinction. Possibly the small dust grains have been destroyed by the GRBs themselves. Our findings support the idea that primarily optically selected QSOs probe mainly low gas/dust regions of high redshift galaxies, while the more powerful GRBs can be detected through denser regions of their ISM (molecular clouds and star forming regions). Therefore GRB-DLAs and QSO-DLAs together provide a more complete picture of the global properties of the interstellar medium in high redshift galaxies.