Quasar-galaxy correlations: A search for amplification bias

Weak gravitational lensing by clusters and superclusters can change our perception of the distribution of bright background quasars through a phenomenon known as amplification 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 homogeneous sample of 32 CCD fields centered on quasars that are bright both at radio (S_{5 GHz} > 1 Jy) and optical (V < 18.5) wavelengths, along with 11 control fields. These 69′ × 69′ fields are a small subset of our larger survey of the fields around approximately 100 quasars and approximately 20 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 have detected a correlation between galaxy number density and angular distance from bright quasars on scales less than 30′ with a significance of 99%. This angular correlation is consistent with the recent semianalytic models of Dolag & Bartelmann that incorporate nonlinear evolution of matter.