Flux Ratio Anomalies: Micro- and Milli-lensing

Simple models for lensing potentials that successfully reproduce the positions of quadruple images to high accuracy fail abysmally in reproducing the flux ratios of the multiple images, suggesting the presence of small scale structure within the lensing galaxies. It has been argued that the flux ratio anomalies observed at radio wavelengths signal the presence of CDM mini-halos. We argue that at least some of the anomalies observed at optical wavelengths result from micro-lensing by stars. This model succeeds in explaining observed asymmetry between minima and saddle-points of the light travel time only if a substantial fraction of the projected mass is in a smooth, dark component.