What are the environments of lens galaxies?

Using measured tangential shear profiles and number counts of massive elliptical galaxies, the halo occupation distribution of strong lensing galaxies is constrained. The resulting HOD is then used to populate an N-body simulation with lens galaxies, in order to assess the importance of environment for strong lensing systems. Typical estimated values for the convergence and shear produced by nearby correlated matter are kappa = gamma = 0.03, with much stronger events occurring relatively infrequently. This implies that estimates of quantities like the Hubble constant are not expected to be significantly biased by environmental effects. One puzzle is that predicted values for the external shear at lens galaxies are far below the values obtained by modeling of strong lensing data.