Using the 2dF galaxy redshift survey to detect gravitationally-lensed quasars

Abstract

Galaxy redshift surveys can be used to detect gravitationally-lensed quasars if the spectra obtained are searched for the quasars' emission lines. Previous investigations of this possibility have used simple models to show that the 2 degree Field (2dF) redshift survey could yield several tens of new lenses, and that the larger Sloan Digital Sky Survey should contain an order of magnitude more. However the particular selection effects of the samples were not included in these calculations, limiting the robustness of the predictions; thus a more detailed simulation of the 2dF survey was undertaken here. The use of an isophotal magnitude limit reduces both the depth of the sample and the expected number of lenses, but more important is the Automatic Plate Measuring survey's star-galaxy separation algorithm, used to generate the 2dF input catalogue. It is found that most quasar lenses are classed as merged stars, with only the few lenses with low-redshift deflectors likely to be classified as galaxies. Explicit inclusion of these selection effects implies that the 2dF survey should contain 10 lenses on average. The largest remaining uncertainty is the lack of knowledge of the ease with which any underlying quasars can be extracted from the survey spectra.