Modelling of the Complex CASSOWARY/SLUGS Gravitational Lenses
  We present the first high-resolution images of CSWA 31, a gravitational lens
system observed as part of the SLUGS (Sloan Lenses Unravelled by Gemini
Studies) program. These systems exhibit complex image structure with the
potential to strongly constrain the mass distribution of the massive lens
galaxies, as well as the complex morphology of the sources. In this paper, we
describe the strategy used to reconstruct the unlensed source profile and the
lens galaxy mass profiles. We introduce a prior distribution over
multi-wavelength sources that is realistic as a representation of our knowledge
about the surface brightness profiles of galaxies and groups of galaxies. To
carry out the inference computationally, we use Diffusive Nested Sampling, an
efficient variant of Nested Sampling that uses Markov Chain Monte Carlo (MCMC)
to sample the complex posterior distributions and compute the normalising
constant. We demonstrate the efficacy of this approach with the reconstruction
of the group-group gravitational lens system CSWA 31, finding the source to be
composed of five merging spiral galaxies magnified by a factor of 13.
