SIMBAD references

The Jodrell Bank-VLA Astrometric Survey (JVAS) and the Cosmic Lens All Sky Survey (CLASS) have been systematically searched for multiple gravitational imaging of sources with image separations between 6arcsec and 15arcsec, associated with galaxy group and cluster lensing masses. The radio and optical follow-up observations of all candidates are presented. From a total of ∼15000 sources only one weak candidate remains and this is not contained in the statistically complete sample of flat-spectrum JVAS/CLASS sources of 11670 sources. A simple Press-Schechter analysis is performed. For singular isothermal sphere lenses the lack of multiple image systems is inconsistent with the currently favoured cosmologies with (Ω_M,Ω_Λ)=(0.3,0.7) at the 4.2σ level. Cored isothermal lenses reduce the expected number of lens systems and we suggest that the most probable interpretation of our results is that the surface mass density of groups and clusters of galaxies is not high enough to cause multiple imaging and the presence of the mass concentrations associated with individual galaxies is required to produce image separations such as those in B0957+561.