Intervening Metal Systems in GRB and QSO sight - lines : The Mg ii and C iv Question

Prochter et al. 2006 recently found that the number density of strong intervening 0.5 < z < 2 Mgii absorbers detected in gamma-ray burst (GRB) afterglow spectra is nearly 4 times larger than in QSO spectra. We have conducted a similar study using Civ absorbers. Our Civ sample, consisting of a total of 19 systems, is drawn from 3 high resolution and high to moderate S/N VLT/UVES spectra of 3 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 with the same quantities measured in QSO spectra. We discuss several possibilities for the discrepancy between Civ and Mgii absorbers and conclude that a higher dust extinction in the Mgii QSO samples studied up to now would give the most straightforward solution. However, this effect is only important for the strong Mgii 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 Based on observations collected at the European Southern Observatory, Chile; proposal no. 75.A-0385, 75.A-0603, 77.D-0661 Physics Department, Guilford College, Greensboro, North Carolina Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, D-85748 Garching bei München, Germany European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile ar X iv :0 70 5. 07 06 v2 [ as tr oph ] 5 J ul 2 00 7