Compound gravitational lensing as a probe of dark matter substructure within galaxy halos

We show how observations of multiply imaged quasars at high redshift can be used as a probe of dark matter clumps (subhalos with masses less than or similar to 10(9) M-.) within the virialized extent of more massive lensing halos. A large abundance of such satellites is predicted by numerical simulations of galaxy formation in cold dark matter (CDM) cosmologies. Small-scale structure within galaxy halos affects the flux ratios of the images without appreciably changing their positions. We use numerical simulations to quantify the effect of dark matter substructure on the distribution of magnifications and find that the magnification ratio of a typical image pair will deviate significantly from the value predicted by a smooth lensing potential if, near the Einstein radius, only a few percent of the lens surface density is contained in subhalos. The angular size of the continuum source dictates the range of subclump masses that can have a detectable effect : to avoid confusion with gravitational microlensing caused by stars in the lens galaxy, the background source must be larger than the optical-continuum-emitting region of a QSO. We also find that substructure will cause distortions to images on milliarcsecond scales and bias the distribution of QSO magnification ratios-two other possible methods of detection.