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

Abstract

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.