Testing the uniqueness of gravitational lens mass models
  The positions of images produced by the gravitational lensing of background
sources provide unique insight in to galaxy-lens mass distribution. However,
even quad images of extended sources are not able to fully characterize the
central regions of the host galaxy. Most previous work has focused either on
the radial density profile of the lenses or localized substructure clumps.
Here, we concentrate on the azimuthal mass asymmetries near the image circle.
The motivation for considering such mass inhomogeneities is that the transition
between the central stellar dominated region and the outer dark matter
dominated region, though well represented by a power law density profile, is
unlikely to be featureless, and encodes information about the dynamical state
and assembly history of galaxies. It also happens to roughly coincide with the
Einstein radius. We ask if galaxies that have mass asymmetries beyond
ellipticity can be modeled with simpler lenses, i.e., can complex mass
distributions masquerade as simple elliptical+shear lenses? Our preliminary
study indicates that for galaxies with elliptical stellar and dark matter
distributions, but with no mass asymmetry, and an extended source filling the
diamond caustic, an elliptical+shear lens model can reproduce the images well,
thereby hiding the potential complexity of the actual mass distribution. For
galaxies with non-zero mass asymmetry, the answer depends on the size and
brightness distribution of the source, and its location within the diamond
caustic. In roughly half of the cases we considered the mass asymmetries can
easily evade detection.
