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 wqg for two complete samples of radio-loud quasars, the southern 1 Jy and half-Jansky samples. The existence of a positive correlation between z ∼ 1 quasars and z ≈ 0.15 galaxies is confirmed at a p = 99.0 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 wqg for incomplete samples. This effect may explain many of the surprisingly strong quasar—galaxy associations found in the literature. Nevertheless, the value of wqg 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.