The importance of intergalactic structure to gravitationally lensed quasars

Image flux ratio anomalies have been attributed to substructures within the gravitational lens and to small-mass halos (M less than or similar to 10(10) M-circle dot) in intergalactic space. In this paper, analytic calculations are presented that help in the understanding of how intergalactic halos affect magnification ratios. It is found that intergalactic halos can produce anomalies at a level similar to those that are observed. Intergalactic halos with masses < 10(10) M-circle dot are expected to cause relative deflections between images of order 10 mas, which are then magnified by the primary lens. They will also cause fluctuations in the surface density on the several percent level. The importance of intergalactic halos depends strongly on the radial profile of the halos and the primordial power spectrum at small scales. Strongly lensed quasars provide an opportunity to probe these properties. A strong dependence on the QSO redshift is predicted and can be used to distinguish between intergalactic structure and substructure as the cause of magnification anomalies. This analytic approach also explains why some previous semianalytic estimates disagreed with numerical calculations.