Introduction

Gravitationally-lensed quasars are very valuable. Individual lenses can be used to constrain the properties of the source, the distribution of mass in the deflector, and, through microlensing, the composition of the deflector. The frequency of lensing can be used to place limits on the cosmological model, and the distribution of image separations provides a useful probe of the average deflector mass. These and other uses of lenses are well known, but cases of multiple imaging are still rare ( known to date). This is not due to any lack of effort on the part of observers - most lenses are discovered by dedicated re-observation of previously identified high-redshift sources, but the likelihood of lensing is sufficiently low that imaging of up to fields is required per lensing event.

It is also possible to search for lenses spectroscopically, as opposed to morphologically. A small fraction of quasars are multiply-imaged by galaxies sufficiently nearby to make the composite object appear extended, whilst the quasar is bright enough for its emission lines to be apparent in the composite spectrum. Only one quasar lens - Q 2237+0305 (Huchra et al. 1985) - has so far been discovered in this manner, but the proximity of the deflector (a redshift 0.04 spiral galaxy) has allowed a number of unique measurements to be made, as summarised by Mortlock & Webster (2000). Spectroscopic lens surveys will primarily be sensitive to these particularly useful systems, but the restriction that the deflector be so nearby also means that the frequency of such lenses is considerably lower than that of the general lens population (Kochanek 1992; Mortlock & Webster 2000). Thus a dedicated spectroscopic lens survey would be disastrously inefficient, but galaxy redshift surveys (GRSs) provide large samples of galaxy spectra as a matter of course. Such lens surveys are potentially very efficient, with the only data requirements being follow-up imaging of the best spectroscopic candidates.

Kochanek (1992) and Mortlock & Webster (2000) have already investigated the statistical likelihood of finding lensed quasars in redshift surveys. The 2 degree Field (2dF) GRS (Folkes et al. 1999) should contain at least ten lenses amongst its

spectra¹, and the larger Sloan Digital Sky Survey (SDSS; York et al. 2000), with close to 10⁶ galaxies, may yield up to a hundred spectroscopic lenses.

These results are extended to the near-infrared-selected 6 degree Field (6dF) GRS here. The survey is described in Section 2, and the lens calculation is presented in Section 3. The results are summarised in Section 4.

© Copyright Astronomical Society of Australia 1997