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 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 A _ { V } \approx 1 mags , to be compared with typical A _ { V } \lesssim 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 .