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Lenses Monitoring
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
Links
The FIRST Survey
The APM Catalog
The PMM USNO-A Catalogue
Constructed: December 1999, Eran Ofek