Analysis of luminosity distributions and shape parameters of strong gravitational lensing elliptical galaxies

Context. The luminosity profiles of galaxies acting as strong gravitational lenses can be tricky to study. Indeed, strong gravitational lensing images display several lensed components, both point-like and diffuse, around the lensing galaxy. Those objects restrain the study of the galaxy luminosity to its inner parts. Therefore, the usual fitting methods perform rather badly on such images. Previous studies of strong lenses luminosity profiles using such codes and various PSF-determining methods have resulted in somewhat discrepant results.
Aims. The present work aims at investigating the causes of those discrepancies, as well as at designing more robust techniques to study the morphology of early-type lensing galaxies, with the ability to subtract lensed signal from their luminosity profiles. Methods. Each shape parameter, namely, the position angle, ellipticity and half-light radius of the galaxy, are determined as independently from each other as possible. The half-light radius measurement method is based on the computation of isophotes. Its robustness regarding various specific aspects of gravitational lensing image processing is analysed and tested versus that of a widely used fitting code, GALFIT. Those techniques are then applied on a sample of systems from the CASTLES database.
Results. Simulations show that, when restricted to small, inner parts of the lensing galaxy, the technique presented here is more robust than GALFIT. It gives unbiased results, while GALFIT leads to an overestimation of the half-light radius that can reach about 10%, depending, among others, on the SNR. It is therefore better-suited than GALFIT for gravitational lensing images. It is also able to study lensing galaxies that are not much larger than the PSF, as opposed to GALFIT. New values for the half-light radius of the objects in our sample are presented and compared to previous works.