The shape and size of a galaxy-mass dark matter halo can only be measured using a luminous tracer, such as stars, gas or photons. In the last case, gravitational lensing of extended background sources provides many paths for photons to sample the gravitational potential of the lensing galaxy. Multiple independent light paths provide tighter constraints on the mass profile of the lens galaxy than is possible from single path sampled by the lensed images of unresolved sources.

Using software specifically developed to model resolved images, we analyse the optical and radio Einstein ring systems 0047-2808 and MG1549+3047. For 0047-2808, elliptical pseudo-isothermal models (e.g. SIE, PIEP) are able to reproduce the lensed image well, with a surface mass power-law $\Sigma \propto r^{-\gamma}$, best fitted by $\gamma = 1.07 \pm 0.03$, which is slightly steeper than isothermal. Using a stars-plus-halo model, we find the projected halo is rounder than the visible galaxy (axis ratio $0.82 \pm 0.01$) and aligned closely with the stars (offset $8^{\circ} \pm 1$). Preliminary results for MG1549+3047 also rule out the constant M/L model. A stars + PIEP model suggests a very round (axis ratio $\ge 0.98$) halo.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html