Lens galaxy environments and anomalous flux ratios in gravitational lenses

The fraction of substructures required to account for anomalous flux ratios in gravitational lens systems appears to be higher than that predicted in the standard cold dark matter cosmology. We present a possible alternative route to anomalous flux ratios from lens galaxy environments. We consider compound lens systems where a lens galaxy lies in a group or cluster, and estimate the contribution of substructures in the group/cluster to the fraction using an analytic model of substructures. We find that the contribution becomes dominant when the impact parameter of the lens is less than similar to 30 per cent of the virial radius of the group/cluster. This indicates that environmental effects can partly explain the high incidence of anomalous flux ratios.