Statistical lensing of faint QSOs by galaxy clusters

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Abstract
We investigate the anti-correlation between faint high redshift QSOs and low redshift galaxy groups found by Boyle, Fong & Shanks (1988), on the assumption that it is caused by gravitational lensing of a flat QSO number count, rather than any other systematic effect (e.g dust). An isothermal sphere lens model, requires a velocity dispersion of 1286+72-91 km/s. Addition of a constant density plane reduces this to 1143+109-153 km/s, while the plane density is 0.081+-0.032h g/cm**2. Both these values are considerably larger than the 400-600km/s expected for poor clusters and groups and imply that the mass associated with such groups is ~4 times larger than inferred from virial analyses. If due to lensing, this measurement clearly tends to favour high values of Omega_0. We demonstrate how an estimate of Omega_0 may be obtained, finding a simple scaling relation for Omega_0. Systematic uncertainties in the extent of the anti-correlation and the group space density currently dominate this determination, but this will be a potential route to estimating omega_0 in improved galaxy-QSO datasets where these systematics can be better controlled. We compared our result to that of Williams & Irwin (1998) who find a positive correlation between bright LBQS QSOs and APM galaxies. Adapting the analysis of Williams & Irwin to our use of galaxy groups, we find agreement between the amplitude of the positive cross-correlation found for bright QSOs and the amplitude of the negative cross-correlation found for faint QSOs. This analysis leads to a common estimate of Omega_0*sigma_8~3-4. This, however, is significantly higher than indicated from several other analyses. Further tests of the accuracy of galaxy-QSO cross-correlations and their implications for Omega_0 and sigma_8 will soon be available from the new 2dF QSO catalogue.

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