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A Survey for Large Separation Lensing
Among FIRST Quasars

Eran O. Ofek , Dan Maoz , Francisco Prada , Tsafrir Kolatt & Hans-Walter Rix

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 (7"-30") image separations, which probe masses on the scale of galaxy clusters. We have carried out a survey for gravitationally-lensed objects, among sources in the FIRST 20cm radio survey that have unresolved blue optical counterparts in the APM digitization of the Palomar Observatory Sky Survey. From the statistics of ongoing surveys that search for quasars among FIRST sources, we estimate that there are about 9100 quasars in this source sample. A second source list was created by cross-correlating a sub-sample of the FIRST catalog with the USNO-A1.0 digitization of the POSS, but without the resolution and color criteria. This subsample serves to check that a population of red quasars, or point-sources misclassified by the APM as galaxies, have not been missed. Using broad-band imaging, we have isolated all objects with double radio components separated by 5"-30", that have corresponding optical counterparts which are unresolved, and which have similar BVI colors. Our criteria for similar colors conservatively allow for observational error and for differences due to color variations combined with time delays between lensed images. Spectroscopy of these candidates shows that none of the pairs are lensed quasars. Our procedure is novel in that our exhaustive search for lensed objects does not require prior identification as such of the quasars in the sample, or (for the second source list) knowledge of their spectral properties. This is the largest lensing survey to date, and our null result indicates that strong lensing of background point sources by clusters is extremely rare. Characterization of the FIRST-selected quasar population will enable using our result to constrain the mass properties of clusters.

Paper, Catalogue and Tables

 Paper I: A Survey for Large Separation Lensed FIRST Quasars
    Catalogue of APM optical Counterparts to the FIRST catalog (Version Jul, 21st 1999)
    Catalogue of APM optical Counterparts to the FIRST catalog (New objects in Version Jul, 05th 2000)
    Candidates BVI Observations - ASCII Table
 Poster: A Survey for Large Separation Lensing Among 9100 FIRST Quasars (ppt file)
 Paper II: A Survey for Large Separation Lensed FIRST Quasars, II. Magnification Bias and Redshift Distribution


 The FIRST Survey
 The APM Catalog
 The PMM USNO-A Catalogue

Constructed: December 1999, Eran Ofek