EVIDENCE FOR SUBSTRUCTURE IN LENS GALAXIES

We discuss whether one should expect that multiply imaged QSOs can be understood with 'simple' lens models that contain only a few parameter s. Whereas for many lens systems such simple mass models yield a remar kably good description of the observed properties, there are some syst ems which are notoriously difficult to understand quantitatively. We a rgue that at least in one case (B 1422 + 231) these difficulties are n ot (solely) due to a 'wrong' parametrization of the lens model, but th at the discrepancy between observed and model-predicted flux ratios is due to substructure in the lens. As in microlensing for optical fluxe s, such substructure can distort also the radio flux ratios predicted by 'simple' mass models, in particular for highly magnified images, wi thout appreciably changing image positions. Substructure also does not change the time delay significantly, and therefore has little effect on the determination of the Hubble constant using time delays. We quan tify these statements with several simple scenarios for substructure, and propose a strategy to model lens systems in which substructure is suspected.