Dissecting a galaxy: mass distribution of 2237+0305

We determine the mass distribution of a spiral galaxy, 2237+0305 using both gravitational lensing and dynamical constraints. We find that lensing can break the disc-halo degeneracy. 2237+0305 has a sub-maximal disc, contributing 57±3 per cent of the rotational support at the disc maximum. The disc mass-to-light ratio is 1.1±0.2 in the I-band and the bulge, 2.9±0.5. The dark matter halo, modelled as a softened isothermal sphere, has a large core radius (13.4±0.4 kpc ≡ 1.4rd) to high accuracy for the best-fit solution. The image positions are reasonably well fitted, but require further rotation information to obtain a unique solution.