Quasar-galaxy correlations: A detection of magnification bias

Weak gravitational lensing by clusters and superclusters can change our per ception of the distribution of bright background quasars through the phenom enon of magnification bias. It has been suggested that this lensing will le ad to an association of moderate-redshift quasars (z similar to 1.0-2.0) an d foreground galaxies (z similar to 0.2-0.3). We report on the analysis of a sample of 90 CCD fields centered on moderate-redshift quasars that are br ight at both radio (S-5 (GHz) > 1 Jy) and optical (V < 21.0) wavelengths, a long 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 cor relation test, we detect a correlation between galaxy number density and an gular distance from bright quasars on scales of <similar to>15' with a sign ificance of 96%. The observed correlation is unlikely the result of Galacti c dust and is consistent with weak gravitational lensing of the quasars by foreground matter. We also determine the two-point angular correlation func tion between these background quasars and foreground galaxies. We find a co rrelation amplitude of 0.056 +/- 0.020 over scales of similar to5'. This an gular correlation is consistent with the semianalytic weak gravitational le nsing models of Dolag & Bartelmann (published in 1997), which incorporate n onlinear evolution of mass clustering.