THE HUBBLE SPACE TELESCOPE SNAPSHOT SURVEY .4. A SUMMARY OF THE SEARCH FOR GRAVITATIONALLY LENSED QUASARS

We report the concluding results of the HST Snapshot Survey for gravit ationally lensed quasars. New observations of 153 high-luminosity z > 1 quasars are presented, bringing to 498 the total number of quasars o bserved in the survey. The new observations do not reveal new candidat es for gravitational lensing. We present tables summarizing all of the snapshot observations, with measured V-magnitudes, accurate to 0.1 ma g, for each of the quasars successfully observed. The tables are avail able on request in machine-readable form. The observed frequency of le nsing of quasars into multiple images is 3-6 out of 502, depending on whether one counts candidates that are not yet securely confirmed and cases in which clusters play a role. This frequency is in the range pr edicted by calculations with a vanishing cosmological constant, assumi ng galaxies can be modeled by unevolving isothermal spheres dominated in their centers by dark matter. The observed frequency is an order of magnitude lower than expected in such models when the universe is str ongly dominated by a cosmological constant. This conclusion is, howeve r, sensitive to the model assumptions and to the precise number of act ual lensed quasars.