Tests for Substructure in Gravitational Lenses

The flux anomalies in four-image gravitational lenses can be interpreted asevidence for the dark matter substructure predicted by cold dark matter (CDM)halo models. In principle, these flux anomalies could arise from alternatesources such as absorption, scattering or scintillation by the interstellarmedium (ISM) of the lens galaxy, problems in the ellipsoidal macro models usedto fit lens systems, or stellar microlensing. We apply several tests to thedata that appear to rule out these alternate explanations. First, the radioflux anomalies show no significant dependence on wavelength, as would beexpected for almost any propagation effect in the ISM or microlensing by thestars. Second, the flux anomaly distributions show the characteristicdemagnifications of the brightest saddle point relative to the other imagesexpected for low optical depth substructure, which cannot be mimicked by eitherthe ISM or problems in the macro models. Microlensing by stars also cannotreproduce the suppression of the bright saddle points if the radio source sizesare consistent with the Compton limit for their angular sizes. Third, while itis possible to change the smooth lens models to fit the flux anomalies in somesystems, we can rule out the necessary changes in all systems where we haveadditional lens constraints to check the models. Moreover, the parameters ofthese models are inconsistent with our present observations and expectationsfor the structure of galaxies. We conclude that low-mass halos remain the bestexplanation of the phenomenon.Comment: Submitted to Ap