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We show how observations of multiply-imaged quasars at high redshift can beused as a probe of dark matter clumps (subhalos with masses ~ 10^9 solarmasses) within the virialized extent of more massive lensing halos. A largeabundance of such satellites is predicted by numerical simulations of galaxyformation in cold dark matter (CDM) cosmogonies. Small-scale structure withingalaxy halos affects the flux ratios of the images without appreciably changingtheir positions. We use numerical simulations to quantify the effect of darkmatter substructure on the distribution of magnifications, and find that themagnification ratio of a typical image pair will deviate significantly from thevalue predicted by a smooth lensing potential if, near the Einstein radius,only a few percent of the lens surface density is contained in subhalos. Theangular size of the continuum source dictates the range of subclump masses thatcan have a detectable effect: to avoid confusion with gravitationalmicrolensing caused by stars in the lens galaxy, the background source must belarger than the optical continuum-emitting region of a QSO. We also find thatsubstructure will cause distortions to images on milli-arcsecond scales andbias the distribution of QSO magnification ratios -- two other possible methodsof detection.Comment: accepted for publication in ApJ, 21 pages, 10 figure

Compound Gravitational Lensing as a Probe of Dark Matter Substructure within Galaxy Halos