A survey for large-separation lensed FIRST quasars

Little is known about the statistics of gravitationally lensed quasars at l arge (7-30 arcsec) image separations, which probe masses on the scale of ga laxy clusters. We have carried out a survey for gravitationally lensed obje cts, among sources in the FIRST 20-cm radio survey that have unresolved opt ical counterparts in the digitizations of the Palomar Observatory Sky Surve y. From the statistics of ongoing surveys that search for quasars among FIR ST sources, we estimate that there are about 9100 quasars in this source sa mple, making this one of the largest lensing surveys to date. Using broad-b and imaging, we have isolated all objects with double radio components sepa rated by 5-30 arcsec that have unresolved optical counterparts with similar BVI colours. Our criteria for similar colours conservatively allow for obs ervational error and for colour variations due to time delays between lense d images. Spectroscopy of these candidates shows that none of the pairs are lensed quasars. This sets an upper limit (95 per cent confidence) on the l ensing fraction in this survey of 3.3 x 10(-4) assuming 9100 quasars. Altho ugh the source redshift distribution is poorly known, a rough calculation o f the expected lensing frequency and the detection efficiencies and biases suggests that simple theoretical expectations are of the same order of magn itude as our observational upper limit. Our procedure is novel in that our exhaustive search for lensed objects does not require prior identification of the quasars in the sample as such. Characterization of the FIRST-selecte d quasar population will enable use of our result to constrain quantitative ly the mass properties of clusters.