Investigating lensing by absorbers in the 2dF-Quasar survey

Abstract

We use the first data release of the 2-degree Field Quasar survey to investigate the effect of gravitational magnification by foreground absorbing systems on background quasars. We select two populations of quasars from this sample: one with strong MgII/FeII absorbers and one without. The selection is done in such a way that the two populations have the same redshift distribution and the absorber detection procedure discards possible biases with quasar magnitude. We then compare their magnitude distributions and find a relative excess of bright quasars with absorbers. This effect is detected at the 2.4, 3.7 and 4.4 sigma levels in u-, b- and r-bands. Various explanations of the observed phenomenon are considered and several lines of evidence point towards gravitational lensing causing some of the differences observed in the magnitude distributions. We note that physical quasar-absorber associations may contribute to some extent to the observed correlations for low quasar-absorber velocity differences. We discuss the implications of these findings and propose future work which will allow us to strengthen and extend the results presented here.