Heavy element abundances and dust depletions in 4 Gamma Ray Bursts

We have determined the column densities of UV absorption lines in 4 Gamma Ray Burst (GRB) optical transients, using the curve of growth analysis. From a direct comparison with the column densities of the same elements observed in damped Lyman–α (DLA) systems along QSO sight lines, we unambiguously find that 3 GRB–DLAs are characterized by much higher column densities of ZnII with respect to QSO–DLAs. The relative abundances of [Fe/Zn], [Si/Zn] and [Cr/Zn] as a function of ZnII column density in QSO–DLAs, are smaller at larger N ZnII. These trends are confirmed by the GRB–DLA measurements, being located at the lower [X/Zn] and higher ZnII column part of the QSO– DLA distribution. Very small [Fe/Zn], [Si/Zn] and [Cr/Zn] values in GRBs also indicate a large dust depletion and extinction. Once the dust–to–metals ratios are determined in 3 GRBs, we find A V = 0.6, 0.9 and 1.1 mags, to be compared with typical A V ∼ < 0.1 in QSO–DLAs. All our findings support the idea that QSOs (fainter than GRBs) can only probe low gas/dust regions of high redshift galaxies, while GRBs can be seen through denser regions (molecular clouds and star forming regions) of high redshift galaxies. Therefore GRB–DLAs and QSO– DLAs together provide a more complete picture of the global properties of the interstellar medium in high redshift galaxies.