Quasar-galaxy associations revisited

Gravitational lensing predicts an enhancement of the density of bright, dis tant quasi-stellar objects (QSOs) around foreground galaxies. We measure th is QSO-galaxy correlation w(qg) for two complete samples of radio-loud quas ars, the southern 1 Jy and half-Jansky samples. The existence of a positive correlation between z similar to 1 quasars and z approximate to 0.15 galax ies is confirmed at a p=99.0 per cent significance level (> 99.9 per cent i f previous measurements on the northern hemisphere are included). A compari son with the results obtained for incomplete quasar catalogues (e.g. the Ve ron-Cetty and Veron compilation) suggests the existence of an 'identificati on bias', which spuriously increases the estimated amplitude of w(qg) for i ncomplete 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 whic h could help to explain this discrepancy: galactic dust extinction and stro ng lensing.