Quasar-galaxy correlations and the detection of magnification bias

It has been proposed that gravitational lensing by large-scale structure can lead to an observed correlation of moderate redshift quasars ( z ∼ 0.7–2.0) and foreground galaxies (z ∼ 0.2–0.3) through the phenomenon of magnification bias. In particular, for a sample of quasars with a steep (>0.4) number density slope, magnification bias should result in an overdensity of galaxies in the fields of these quasars.; Here we present the results of a search for the signature of magnification bias in a survey of fields around 90 radio-bright ($S5GHz>1$ Jy) quasars taken from the 1 Jy radio survey. We have detected a 2.5σ overdensity of galaxies in these quasar fields a scale of ∼5′ . We demonstrate that this correlation is consistent with what is expected due magnification bias and is not consistent with overdensities caused by dust obscuration locally or in the external galaxies.; The observed correlation is inconsistent with gravitational lensing by Abell richness class = 1 clusters. However, the amplitude and scale of the observed correlation is consistent with recent semi-analytical models of quasar-galaxy correlations due to gravitational lensing by large-scale structure. We compare our results to these CDM models which incorporate non-linear evolution of the matter power spectrum.