A SURVEY FOR LARGE-IMAGE SEPARATION LENSED QUASARS

The statistics of gravitationally lensed quasars with multiple images in the 0.1 ''-7 '' range have been measured in various surveys. Little is known, however, about lensed-quasar statistics at larger image sep arations, which probe masses on the scale of galaxy clusters. We exten d the results of the Hubble Space Telescope (HST) Snapshot Survey for Lensed Quasars to the 7 ''-50 '' range for a subsample of 76 quasars t hat is free of known selection effects. Using a combination of multico lor photometry and spectroscopy, we show that none of the point source s in the entire field of view of the HST observations of these quasars are lensed images. Large-separation quasar lensing is therefore not c ommon. We carry out a detailed calculation of the expected statistics of large-separation lensing for this quasar sample, incorporating real istic input for the mass profiles and mass function of galaxy clusters . We find that the observational null results are consistent with the expected effect of galaxy clusters, even if these have existed in thei r present form and number since z similar to 2 (and certainly if they were formed more recently). The rarity of large-separation lensed quas ars can rule out some extreme scenarios, e.g., that the mass function of clusters has been severely underestimated or that large mass concen trations that are not associated with galaxies (i.e., ''failed'' clust ers) are common. The rareness of cluster lensing also sets limits on t he cosmological constant lambda that are independent of limits derived from galaxy lensing. The lensing frequency depends strongly on the ce ntral density of clusters. The lensing statistics of larger quasar sam ples (e.g., the Sloan Digital Sky Survey) can probe the structure, num ber, and evolution of clusters, as well as the geometry of space.