2007ApJ...669..741S

Prochter and coworkers recently found that the number density of strong intervening 0.5<z<2 Mg II absorbers detected in gamma-ray burst (GRB) afterglow spectra is nearly 4 times larger than those in QSO spectra. We have conducted a similar study using C IV absorbers. Our C IV sample, consisting of a total of 19 systems, is drawn from three high-resolution and high to moderate signal-to-noise ratio VLT UVES spectra of three long-duration GRB afterglows, covering the redshift interval 1.6<z<3.1. The column density distribution and number density of this sample do not show any statistical difference from the same quantities measured in QSO spectra. We discuss several possibilities for the discrepancy between C IV and Mg II absorbers and conclude that a higher dust extinction in the Mg II QSO samples studied up to now would give the most straightforward solution. However, this effect is only important for the strong Mg II absorbers. Regardless of the reasons for this discrepancy, this result confirms once more that GRBs can be used to detect a side of the universe that was unknown before, not necessarily connected with GRBs themselves, providing an alternative and fundamental investigative tool of the cosmic evolution of the universe.