Published online by Cambridge University Press: 26 May 2016
Many quasars that are gravitationally lensed exhibit flux ratio “anomalies” that cannot be explained under the hypothesis that the lensing potential is smooth on scales smaller than 1 kpc. Micro-lensing by stars is a natural source of granularity in the lens potential. the character of the expected fluctuations due to Micro-lensing depends sensitively on the relative surface densities of micro-lenses (stars) and smoothly distributed (dark) matter. Observations of flux ratios may therefore be used to infer the ratio of stellar to dark matter along the line of sight – typically at impact parameters 1.5 times the half light radius. Several recently discovered systems have anomalies that would seem to be explained by Micro-lensing only by demanding that 70-90% of the matter along the line of sight be smoothly distributed.