Strong gravitational lensing by multiple galaxies

We discuss strong gravitational lensing by multiple objects along any line of sight. The probability for strong gravitational lensing by more than one lens is small, but a number of strong lens systems in which more than one separate lens contribute significantly to the lensing potential will be det ected in the large sample of lens systems compiled with new instruments. Us ing multi-lens ray-tracing, we estimate the likelihood for gravitational le nsing by two lenses at different redshifts and investigate typical image ge ometries and magnification cross-sections. We find that, for a cosmology wi th Omega (M) = 0.3 and Omega (Lambda) = 0.7, about one in 20 lens systems c onsists of two lenses with merging caustics. Multiple lens systems differ f rom single lenses as the presence of a second lens in close proximity along the line of sight leads to a strongly asymmetric potential, which increase s the multiple imaging cross-section and significantly changes the image co nfiguration. The external shear induced by a second nearby galaxy, group or cluster can significantly affect image positions even for more widely sepa rated lens pairs. Both of these effects must be accounted for in lens model ling. We also show how the presence of aligned discs in the pair of lensing galaxies can lead to very large high-magnification cross-sections. Lensing by more than one galaxy along the line of sight can lead to interesting im age configurations. Such systems will be important in future, both for cons training lens models of individual systems and for statistical lensing.