Intervening Metal Systems in GRB and QSO sight-lines: The Mgii and Civ Question

TL;DR

This study compares CIV and MgII intervening absorbers in GRB and QSO sight-lines, finding no statistical difference in CIV distributions but a discrepancy in MgII, likely due to dust extinction effects.

Contribution

It provides the first CIV absorber comparison in GRB and QSO spectra, highlighting differences with MgII and discussing potential causes for these discrepancies.

Findings

CIV absorber distributions are similar in GRB and QSO spectra.

MgII absorber densities differ significantly between GRB and QSO spectra.

Dust extinction may explain the MgII discrepancy.

Abstract

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 20 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 GRBs themselves, providing an alternative and fundamental investigative tool of the cosmic evolution of the universe.