We consider the fundamental plane of elliptical galaxies lensed by the gravitational field of a massive deflector (typically, a cluster of galaxies). We show that the fundamental plane relation provides a straightforward measurement of the projected mass distribution of the lens with a typical accuracy of ≈ 0.15 in the dimensionless column density κ. The proposed technique breaks the mass-sheet degeneracy completely and is thus expected to serve as an important complement to other lensing-based analyses. Moreover, its ability to directly measure the mass distribution on the small pencil beams that characterize the size of background galaxies may lead to crucial tests for current scenarios of structure formation.
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