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 Lyα (DLA) systems along QSO sight lines, we find evidence for much higher column densities of Zn II. 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 AV ≈ 1 mag, to be compared with typical AV 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 toward big grains, which would give a gray 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.