Discovering Mass Clumps in Distant Galaxies with Lensing: The Case of B1422+231

Substructure in distant gravitational lens galaxies can be detected because it alters the brightnesses and shapes of the lensed images. The optical and radio flux ratios in the four-image lens B1422+231 imply that there is a \sim 10^4-10^7 Msun/h mass clump in the lens galaxy in front of image A; this is the first constraint on the mass of a particular clump lying in a distant galaxy and detected by its mass. The data also indicate that a small clump, perhaps a star, is passing in front of image B and making the optical flux ratios variable. Both of these hypotheses can be tested with new observations. B1422+231 demonstrates how data at different wavelengths can be used in individual lenses to probe individual mass clumps in distant galaxies.