The 2dF gravitational lens survey
Daniel J. Mortlock, Darren S. Madgwick, Ofer Lahav, PASA, 18 (2), in press.
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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 $\Delta m_{\rm qg}$, which is defined as follows (Kochanek 1992): for a lens in which the galaxy has magnitude $m_{\rm g}$ and the quasar images have total magnitude $m_{\rm q}$, the presence of the quasar emission lines is detectable if $m_{\rm q} \leq m_{\rm g} + \Delta m_{\rm qg}$, but the quasar is undetectable if it is fainter than this. The value of $\Delta m_{\rm qg}$ 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 $\Delta m_{\rm qg} \simeq 4$ for the SDSS spectra, and Mortlock & Webster (2000) used $\Delta m_{\rm qg} \simeq 2$ 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 $\sim 5$ (if $\Delta m_{\rm qg} \simeq 1$) or as high as $\sim 20$ (if $\Delta m_{\rm qg} \simeq 3$). Fortunately the value of $\Delta m_{\rm qg}$ appropriate to a given search technique can be evaluated by analysing simulated composite lens spectra.

Next Section: The 2dF gravitational lens
Title/Abstract Page: The 2dF gravitational lens
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Contents Page: Volume 18, Number 2