GRAVITATIONAL LENSING STATISTICS BASED ON A LARGE-SAMPLE OF HIGHLY LUMINOUS QUASARS

Results on gravitational lensing statistics applied to a sample of 469 highly luminous quasars are reported. The objects were directly imaged, either from the ground (ESO, CFH) under optimal seeing conditions, or using the Hubble Space Telescope. We have derived values for the effectiveness parameter F of galaxies, modeled by means of singular isothermal spheres, to produce macrolensed images of distant quasars, and upper limits on the density parameter OMEGA(L) of compact objects with masses congruent-to 10(10)-10(12)M .. Adopting H0=50 km/s/Mpc, OMEGA0=1 and LAMBDA=0, we find that at the 99.7% confidence level, 0.005 < F < 0.478 and that OMEGA(L) < 0.02. A critical discussion of these results is presented. Finally, comparing the efficiencies of ground-based and space instruments used to search for gravitational lens systems among highly luminous quasars, we conclude that for the near future, ground based direct imaging characterized by a good dynamical range still constitutes the best observational strategy.