Using galaxy redshift surveys to detect gravitationally lensed quasars

Gravitationally lensed quasars can be discovered as a by-product of galaxy redshift surveys. Lenses discovered spectroscopically in this way should re quire less observational effort per event than those found in dedicated len s surveys. Further, the lens galaxies should be relatively nearby, facilita ting a number of detailed observations that are impossible for the more com mon high-redshift lenses. This is epitomized by the wide range of results t hat have been obtained from Q 2237+0305, which was discovered as part of th e Center for Astrophysics redshift survey, and remains the only quasar lens discovered in this way. The likelihood of this survey yielding a lens is c alculated to be similar to0.03, which is an order of magnitude larger than previous estimates due to two effects. First, the quasar images themselves increase the observed flux of the lens, so that lens galaxies up to a magni tude fainter than the nominal survey limit must be included in the calculat ion. Secondly, it is possible for lensed quasars with extremely faint defle ctors to enter the survey due to the extended morphology of the multiple im ages. Extrapolating these results to future surveys, the 2 degree Field gal axy redshift survey should contain between 10 and 50 lenses and the Sloan D igital Sky Survey should yield between 50 and 300 lenses, depending on the cosmological model and the observing conditions.