Density profile, velocity anisotropy and line-of-sight external convergence of SLACS gravitational lenses

TL;DR

This study combines stellar dynamics and gravitational lensing data from 58 SLACS lenses to analyze their mass profiles, velocity anisotropy, and external convergence, revealing an approximately isothermal density profile with degeneracies affecting parameter estimates.

Contribution

It provides a joint Bayesian analysis of lens density profiles, anisotropy, and external convergence, highlighting the degeneracies and robustness of the isothermal profile conclusion.

Findings

Density profile is compatible with an isothermal model.

Degeneracy exists between density slope and velocity anisotropy.

External convergence has a small effect but increases dispersion.

Abstract

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%.