Light propagation in inhomogeneous universes. IV. Strong lensing and environmental effects

We study the gravitational lensing of high-redshift sources in a lambda cold dark matter (LambdaCDM) universe. We have performed a series of ray-tracing experiments and selected a subsample of cases of strong lensing (multiple images, arcs, and Einstein rings). For each case, we identified a massive galaxy that is primarily responsible for lensing and studied how the various density inhomogeneities along the line of sight (other galaxies, background matter) affect the properties of the image. The matter located near the lensing galaxy, and physically associated with it, has a small effect. The background matter increases the magnification by a few percent at most, while nearby galaxies can increase it by up to about 10%. The effect on the image separation is even smaller. The only significant effect results from the random alignment of physically unassociated galaxies, which can increase the magnification by factors of several, create additional images, and turn arcs into rings. We conclude that the effect of environment on strong lensing is negligible in general, and might be important only in rare cases. We show that our conclusion does not depend on the radial density profile of the galaxies responsible for lensing.