Quasar-Galaxy Correlations: A Detection of Magnification Bias

Abstract

Weak gravitational lensing by clusters and superclusters can change our perception of the distribution of bright background quasars through the phenomenon of magnification bias. It has been suggested that this lensing will lead to an association of moderate-redshift quasars (z ~ 1.0–2.0) and foreground galaxies (z ~ 0.2–0.3). We report on the analysis of a sample of 90 CCD fields centered on moderate-redshift quasars that are bright at both radio (S5 GHz > 1 Jy) and optical (V < 21.0) wavelengths, along with 12 control fields. The quasars presented here are chosen to have the highest potential for evidence of this weak lensing effect. We find no evidence for the presence of rich galaxy clusters (Abell richness class 1) in the foreground of these fields. However, using a Spearman rank-order correlation test, we detect a correlation between galaxy number density and angular distance from bright quasars on scales of ~15' with a significance of 96%. The observed correlation is unlikely the result of Galactic dust and is consistent with weak gravitational lensing of the quasars by foreground matter. We also determine the two-point angular correlation function between these background quasars and foreground galaxies. We find a correlation amplitude of 0.056 ± 0.020 over scales of ~5'. This angular correlation is consistent with the semianalytic weak gravitational lensing models of Dolag & Bartelmann (published in 1997), which incorporate nonlinear evolution of mass clustering.