Lensing and Dynamics in Two Simple Steps
  We present a ready-to-use method to constrain the density distribution in
early-type galaxy lenses. Assuming a power-law density profile, then joint use
of the virial theorem and the lens equation yields simple formulae for the
power-law index (or logarithmic density gradient). Any dependence on orbital
anisotropy can be tightly constrained or even erased completely. Our results
rely just on surface brightnesses and line-of-sight kinematics, making
deprojection unnecessary. We revisit three systems that have already been
examined in the literature (the Cosmic Horseshoe, the Jackpot and B1608+656)
and provide our estimates. Finally, we show that the method yields a good
approximation for the density profile even when the true profile is a broken
power-law, albeit with a mild bias towards isothermality.
