Analysis of luminosity distributions and shape parameters of strong
  gravitational lensing elliptical galaxies
  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 limit 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 software such as
GALFIT or IMFITFITS and various PSF-determining methods have resulted in
discrepant results. The present work aims at investigating the causes of those
discrepancies, as well as at designing more robust techniques for studying the
morphology of early-type lensing galaxies with the ability to subtract a lensed
signal from their luminosity profiles. We design a new method to independently
measure each shape parameter, namely, the position angle, ellipticity, and
half-light radius of the galaxy. Our half-light radius measurement method is
based on an innovative scheme for computing isophotes that is well suited to
measuring the morphological properties of gravitational lensing galaxies. Its
robustness regarding various specific aspects of gravitational lensing image
processing is analysed and tested against GALFIT. It is applied to a sample of
systems from the CASTLES database. Simulations show that when restricted to
small, inner parts of the lensing galaxy, the technique presented here is more
trustworthy than GALFIT. It gives more robust results than GALFIT, which shows
instabilities regarding the fitting region, the value of the Sersic index, and
the signal-to-noise ratio. It is therefore better suited than GALFIT for
gravitational lensing galaxies. It is also able to study lensing galaxies that
are not much larger than the PSF. New values for the half-light radius of the
objects in our sample are presented and compared to previous works.
