  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.

Astrophysics