7. SEARCHES FOR GRAVITATIONAL LENSES
The first examples of multiple quasars, rings, and arcs were identified
in observing programs directed toward quite different goals. Since
then, roughly half of the known instances of gravitational lensing
have arisen serendipitously, which makes it difficult to quantify
the incidence of lensing and to draw cosmographic deductions. In this
section we briefly review attempts to search for lenses in uniform,
complete samples of extragalactic sources.
A significant number of lensed objects have been found from radio
surveys. The MIT-Greenbank (MG) survey
consists of 6000 sources found at 5 GHz using a transit telescope.
These were subsequently mapped at the VLA with 0.3" resolution and
those that exhibited unusual compact structure were imaged optically.
Subsequent spectroscopy of the nonstellar objects has yielded two
secure cases of multiple imaging (plus four candidates) and three
Gravitational lenses have also been sought in sources observed with MERLIN
which is now an ideal instrument for finding
candidates with subarcsecond structure. One ring has been found.
Radio ring images of background extended radio sources ought to be
more common in existing surveys than has hitherto been reported
and there is optimism that many more of
these sources, which are promising for measuring the Hubble constant
(cf Section 4.1), will be found.
Q0957+561 and 1830-211 were also found from radio
Existing quasar surveys (e.g.
show that the number of quasars brighter than some luminosity L
varies as L-, where ~ 2.7
down to ~ 19m. As the dominant lens cross sections (due to fold
caustics, cf Section 3.3)
decrease only as the inverse square of the magnification, the majority
of lensed sourcess brighter than 19m ought to be highly magnified
as a result of magnification bias
Section 6.5). This
has so far proved to be the case, and has motivated optical lens
searches among samples of bright quasars.
Several groups (e.g.
have combined to search for lenses in an ESO key
project. CCD images of approximately 400 bright (MV
1) quasars drawn from highly heterogeneous and
ill-defined catalogs have been obtained. Two secure and two good
candidate lenses have so far been identified, as well as two quasar
pairs (cf Section 2.1). Negative
searches in smaller bright quasar samples
have also been reported
Selection criteria are somewhat poorly defined, which leads to difficulties in
drawing quantitative conclusions about the frequency of multiple
imaging (cf Section 5.2.2).
These shortcomings may be partly overcome in the APM survey
which establishes a methodology for
creating a complete sample. Quasar candidates are obtained by
uniformly scanning photographic plates covering a large fraction of
the southern sky. This survey appears to be highly efficient at
rediscovering known quasars
and 2500 quasars
brighter than 20m are believed to be present in the survey. But
the survey may be less efficient at identifying multiple quasars with
separations similar to the angular resolution because, by selecting
unusually elliptical images, it is prejudiced against small
separations and, in particular, five image configurations
One lens candidate with angular separation 5" [Q1429-008
and three quasar pairs have so far been
found using this method. Further success is
dependent upon obtaining spectra of a large number of candidate
A comparatively new search technique uses the Hubble Space Telescope
to make short exposures during gaps in the regular observing program
of high luminosity quasars
354 bright quasars will be surveyed. Simulations show that, despite the
spread function, this search ought to be sensitive to separations as
small as ~ 0.1" and magnitude differences up to ~ 2m. As
of this writing, one lens candidate has been found [Q1208+101