Investigation of gravitational lens mass models

We have previously reported the discovery of strong gravitational lensing b y faint elliptical galaxies using the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope, and here we investigate their potential use fulness in placing constraints on lens mass models. We compare various elli psoidal surface mass distributions, including those with and without a core radius, as well as models in which the mass distributions are assumed to h ave the same axis ratio and orientation as the galaxy light. We also study models that use a spherical mass distribution having various profiles, both empirical and following those predicted by cold dark matter simulations. T hese models also include a gravitational shear term. The model parameters a nd associated errors have been derived by two-dimensional analysis of the o bserved WFPC2 images. The maximum likelihood procedure iteratively converge s simultaneously on the model for the lensing elliptical galaxy and the len sed image components. The motivation for this study was to distinguish betw een these mass models with this technique. However, we find that, despite u sing the full image data rather than just locations and integrated magnitud es, the lenses are fitted equally well with several of the mass models. Eac h of the mass models generates a similar configuration but with a different magnification and cross-sectional area within the caustic, and both of the se latter quantities govern the discovery probability of lensing in the sur vey. These differences contribute to considerable cosmic scatter in any est imate of the cosmological constant, Lambda, using gravitational lenses.