Mass density slope of elliptical galaxies from strong lensing and resolved stellar kinematics

We discuss constraints on the mass density distribution (parametrized as rho alpha r(-gamma)) in early-type galaxies provided by strong lensing and stellar kinematics data. The constraints come from mass measurements at two 'pinch' radii. One 'pinch' radius r(1) = 2.2R(Einst) is defined such that the Einstein (i.e. aperture) mass can be converted into the spherical mass almost independently of the mass-model. Another 'pinch' radius r(2) = R-opt is chosen so that the dynamical mass, derived from the line-of-sight velocity dispersion, is least sensitive to the anisotropy of stellar orbits. We verified the performance of this approach on a sample of simulated elliptical galaxies and on a sample of 15 SLACS lens galaxies at 0.01 <= z <= 0.35, which have already been analysed in Barnabe et al. by the self-consistent joint lensing and kinematic code. For massive simulated galaxies, the density slope gamma is recovered with an accuracy of similar to 13 per cent, unless r(1) and r(2) happen to be close to each other. For SLACS galaxies, we found good overall agreement with the results of Barnabe et al. with a sample-averaged slope gamma = 2.1 +/- 0.05. Although the two-pinch-radii approach has larger statistical uncertainties, it is much simpler and uses only few arithmetic operations with directly observable quantities.