Why Is the Fraction of Four‐Image Radio Lens Systems So High?

We investigate the frequency of two- and four-image gravitational lenssystems in the Jodrell-VLA Astrometric Survey (JVAS) and Cosmic Lens All-SkySurvey (CLASS), and the possible implications for dark matter halo properties.A simple lensing statistics model, which describes lens galaxies as singularisothermal ellipsoids with a projected axis ratio distribution derived from thesurface brightness ellipticities of early-type galaxies in the Coma cluster, isruled 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 radioquads, including external shear fields, mass distributions flatter than thelight, shallow lensing mass profiles, finite core radii, satellite galaxies,and alterations to the luminosity function for faint flat-spectrum radiosources. We find that none of these mechanisms provide a compelling solution tothe quad problem on their own while remaining consistent with otherobservational constraints.Comment: Final version. 27 pages, including 9 figs, minor typos corrected, ApJ in press (June 2001