Galaxy‐Quasar Correlations between APM Galaxies and Hamburg‐ESO QSOs

Abstract

We detect angular galaxy-quasi-stellar object (QSO) cross-correlations between the APM Catalogue galaxies and a preliminary release (consisting of roughly half of the anticipated final catalog) of the Hamburg-ESO Survey for bright QSOs as a function of source QSO redshift using multiple cross-correlation estimators. Each of the estimators yields very similar results, implying that the APM Catalogue and the Hamburg-ESO Survey are both fair samples of the respective true galaxy and QSO populations. Although the signal matches the expectations of gravitational lensing qualitatively, the strength of the measured cross-correlation signal is significantly greater than the cold dark matter (CDM) models of lensing by large-scale structure would suggest. This same disagreement between models and observation has been found in several earlier studies. We estimate our confidence in the correlation detections versus redshift by generating 1000 random realizations of the Hamburg-ESO QSO survey. We detect physical associations between galaxies and low-redshift QSOs at 99% confidence and detect lensing associations at roughly 95% confidence for QSOs with redshifts between 0.6 and 1. Control cross-correlations between Galactic stars and QSOs show no signal. Finally, the overdensities (underdensities) of galaxies near QSO positions relative to those lying roughly 135'-150' away are uncorrelated with differences in Galactic extinction between the two regions, implying that Galactic dust is not significantly affecting the QSO sample.