Quasar-Galaxy and Galaxy-Galaxy Cross-Correlations: Model Predictions with Realistic Galaxies

Several measurements of QSO-galaxy correlations have reported signals much larger than predictions of magnification by large-scale structure. We find that changes by a factor of 2-10 in the expected signal arise naturally on arcminute scales if the galaxy distribution is modeled realistically, as opposed to a linearly biased version of the mass distribution. Thus the resolution of the measured excess signal may lie in examining the halo occupation properties of the galaxy population sampled by a given survey; this is also the primary information such measurements will provide. We use the halo model of clustering and simulations to predict the magnification induced cross-correlations and errors for forthcoming surveys. With the full Sloan Digital Sky Survey the statistical errors will be below 1 percent. Thus accurate constraints on parameters of the galaxy halo occupation distribution can be obtained from small scale measurements and on the bias parameter from large scales. Since the lensing induced cross-correlation measures the first moment of the halo occupation number of galaxies, these measurements can provide the basis for interpreting galaxy clustering measurements which can measure only the second and higher order moments.