Quasar–galaxy associations revisited

Gravitational lensing predicts an enhancement of the density of bright, distant quasi-stellar objects (QSOs) around foreground galaxies. We measure this QSO–galaxy correlation w _qg for two complete samples of radio-loud quasars, the southern 1 Jy and half-Jansky samples. The existence of a positive correlation between     quasars and     galaxies is confirmed at a     per cent significance level (>99.9 per cent if previous measurements on the northern hemisphere are included). A comparison with the results obtained for incomplete quasar catalogues (e.g. the Veron-Cetty and Veron compilation) suggests the existence of an 'identification bias', which spuriously increases the estimated amplitude of w _qg for incomplete samples. This effect may explain many of the surprisingly strong quasar–galaxy associations found in the literature. Nevertheless, the value of w _qg that we measure in our complete catalogues is still considerably higher than the predictions from weak lensing. We consider two effects which could help to explain this discrepancy: galactic dust extinction and strong lensing.