Density profile and line-of-sight mass contamination of SLACS gravitational lenses

We use data from 58 strong lensing events surveyed by the Sloan Lens ACS Survey to estimate the projected galaxy mass inside their Einstein radii by two independent methods: stellar dynamics and strong gravitational lensing. Four models are examined testing the galaxy-lens density profile and a possible line-of-sigh (l.o.s.) mass contamination. We test a fixed isothermal profile and a power-law with free index, both without and with contamination. For each model, a likelihood analysis is performed to find the parameters that produce the best agreement between the dynamical and lensing masses, and the parameters confidence levels. The Bayesian evidence is calculated to allow a comparison among the models. We find that there is evidence for a l.o.s. mass contamination of the order of 10%, and that the preferred density profile is close to an isothermal profile, ρ ∝ r. If the l.o.s. contamination is neglected, the density profile determined by a joint lensing and dynamical analysis is flatter than isothermal. Subject headings: dark matter — galaxies: elliptical and lenticular, cD — galaxies: kinematics and dynamics — galaxies: structure — galaxies: fundamental parameters — gravitational lensing