Cosmological Information from Quasar-Galaxy Correlations induced by Weak Lensing

The magnification bias of large-scale structures, combined with galaxy biasing, leads to a cross-correlation of distant quasars with foreground galaxies on angular scales of the order of arc minutes and larger. The amplitude and angular shape of the cross-correlation function wQG contain information on cosmological parameters and the galaxy bias factor. While the existence of this cross-correlation has firmly been established, existing data did not allow an accurate measurement of wQG yet, but wide area surveys like the Sloan Digital Sky Survey now provide an ideal database for measuring it. However, wQG depends on several cosmological parameters and the galaxy bias factor. We study in detail the sensitivity of wQG to these parameters and develop a strategy for using the data. We show that the parameter space can be reduced to the bias factor b̄, Ω0 and σ8, and compute the accuracy with which these parameters can be deduced from SDSS data. Under reasonable assumptions, it should be possible to reach relative accuracies of the order of 5%–15% for b̄, Ω0, and σ8. This method is complementary to other weak-lensing analyses based on cosmic shear.