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.
@article{arxiv.1209.2554,
title = {Lensing and Dynamics in Two Simple Steps},
author = {A. Agnello and M. W. Auger and N. W. Evans},
journal= {arXiv preprint arXiv:1209.2554},
year = {2015}
}