Density profile, velocity anisotropy and line-of-sight external
  convergence of SLACS gravitational lenses
  Data from 58 strong-lensing events surveyed by the Sloan Lens ACS Survey are
used to estimate the projected galaxy mass inside their Einstein radii by two
independent methods: stellar dynamics and strong gravitational lensing. We
perform a joint analysis of these two estimates inside models with up to three
degrees of freedom with respect to the lens density profile, stellar velocity
anisotropy, and line-of-sight (LOS) external convergence, which incorporates
the effect of the large-scale structure on strong lensing. A Bayesian analysis
is employed to estimate the model parameters, evaluate their significance and
compare models. We find that the data favor Jaffe's light profile over
Hernquist's, but that any particular choice between these two does not change
the qualitative conclusions with respect to the features of the system that we
investigate. The density profile is compatible with an isothermal, being
sightly steeper and having an uncertainty in the logarithmic slope of the order
of 5% in models that take into account a prior ignorance on anisotropy and
external convergence. We identify a considerable degeneracy between the density
profile slope and the anisotropy parameter, which largely increases the
uncertainties in the estimates of these parameters, but we encounter no
evidence in favor of an anisotropic velocity distribution on average for the
whole sample. An LOS external convergence following a prior probability
distribution given by cosmology has a small effect on the estimation of the
lens density profile, but can increase the dispersion of its value by nearly
40%.
