A Survey for Large–Image Separation Lensed Quasars

The statistics of gravitationally lensed quasars with multiple images in the0.1''-7'' range have been measured in various surveys. Little is known,however, about lensed-quasar statistics at larger image separations, whichprobe masses on the scale of galaxy clusters. We extend the results of the HSTSnapshot Survey for Lensed Quasars to the 7''-50'' range for a sub-sample of 76quasars that is free of known selection effects. Using a combination ofmulticolor photometry and spectroscopy, we show that none of the point sourcesin the entire field of view of the HST observations of these quasars are lensedimages. Large-separation quasar lensing is therefore not common. We carry out adetailed calculation of the expected statistics of large-separation lensing forthis quasar sample, incorporating realistic input for the mass profiles andmass function of galaxy clusters. We find that the observational null resultsare consistent with the expected effect of galaxy clusters, even if these haveexisted in their present form and number since z of about 2. The rarity oflarge-separation lensed quasars can rule out some extreme scenarios, e.g. thatthe mass-function of clusters has been severely underestimated, or that largemass concentrations that are not associated with galaxies (i.e. ``failed''clusters) are common. The rareness of wide lensing also sets limits on thecosmological constant that are independent of limits derived from galaxylensing. The lensing statistics of larger quasar samples can probe thestructure, number, and evolution of clusters, as well as the geometry of space.Comment: LaTex, ApJ, submitte