Lens statistics

Kochanek (1992) and Mortlock & Webster (2000) applied standard techniques to determine the rate of lensed quasars expected in a generic GRS, and a more detailed calculation verified these results for the 2dF survey (Mortlock & Webster 2001). The number of lenses depends principally on two factors: the depth of the survey and the quality of the spectra. The latter, potentially a somewhat ambiguous notion, can be characterised in terms of

, which is defined as follows (Kochanek 1992): for a lens in which the galaxy has magnitude and the quasar images have total magnitude , the presence of the quasar emission lines is detectable if

, but the quasar is undetectable if it is fainter than this. The value of

clearly increases with the integration time of the observations, but is also critically dependent on the properties of galaxy and quasar spectra. Kochanek (1992) estimated

for the SDSS spectra, and Mortlock & Webster (2000) used

for the lower quality 2dF spectra. These figures then led to the estimate that the 2dF GRS should yield about 10 lensed quasars (Mortlock & Webster 2000), although it could be as low as (if

) or as high as (if

). Fortunately the value of

appropriate to a given search technique can be evaluated by analysing simulated composite lens spectra.

á© Copyright Astronomical Society of Australia 1997