Fitting Gravitational Lenses: Truth or Delusion

The observables in a strong gravitational lens are usually just the image positions and sometimes the flux ratios. We develop a new and simple algorithm which allows a set of models to be fitted exactly to the observations. Taking our cue from the strong body of evidence that early-type galaxies are close to isothermal, we assume that the lens is scale-free with a flat rotation curve. However, our algorithm allows full flexibility as regards the angular structure of the lensing potential. Importantly, all the free parameters enter linearly into the model and so the lens and flux ratio equations can always be solved by straightforward matrix inversion. We use this new algorithm to examine some of the claims made for substructure on the basis of "anomalous flux ratios". We demonstrate by explicit construction that some of the lenses for which substructure has been claimed can be well-fit by smooth lens models. This is especially the case when the systematic errors in the flux ratios (caused by microlensing or differential extinction or scatter-broadening) are taken into account. However, there is certainly one system (B1422+231) for which the existing smooth models are definitely inadequate and for which substructure may be implicated. Within a few tens of kpc of galaxy centres, dynamical friction is efficient at dissolving any substructure. So, little substructure lies projected within the Einstein radius of most strong lenses. The numbers of strong lenses for which substructure is currently being claimed may be so large that this contradicts rather than supports cold dark matter theories.