9 v 2 2 9 Ja n 20 01 Why is the Fraction of Four - Image Radio Lens Systems So High ?

We investigate the frequency of two-and four-image gravitational lens systems in the JVAS and CLASS radio surveys, and the possible implications for dark matter halo properties. A simple lensing statistics model, which describes lens galaxies as singular isothermal ellipsoids with a projected axis ratio distribution derived from the surface brightness ellipticities of early-type galaxies in the Coma cluster, is ruled out at the 98% level since it predicts too few four-image lenses (quads). We consider a range of factors that may be increasing the frequency of radio quads, including external shear fields, mass distributions flatter than the light, shallow lensing mass profiles, finite core radii, satellite galaxies, and alterations to the luminosity function for faint flat-spectrum radio sources. We find that none of these mechanisms provides a compelling solution to the quad problem on its own while remaining consistent with other observational constraints.