Stars and Dark Matter in the Spiral Gravitational Lens 2237 +

We construct a mass model for the spiral lens galaxy 2237+0305, at redshift zl=0.04, based on gravitational-lensing constraints, HI rotation, and new stellarkinematic information, based on data taken with the ESI spectrograph on the 10m Keck-II Telescope. High resolution rotation curves and velocity dispersion profiles along two perpendicular directions, close to the major and minor axes of the lens galaxy, were obtained by fitting the Mgb-Fe absorption line region. The stellar rotation curve rises slowly and flattens at r ∼ 1.5 (∼1.1 kpc). The velocity dispersion profile is approximately flat. A combination of photometric, kinematic and lensing information is used to construct a mass model for the four major mass components of the system — the dark matter halo, disc, bulge, and bar. The best-fitting solution has a dark matter halo with a logarithmic inner density slope of γ=0.9±0.3 for ρDM ∝ r −γ , a bulge with M/LB=6.6±0.3Υ⊙, and a disc with M/LB =1.2±0.3Υ⊙, in agreement with measurements of late-type spirals. The bulge dominates support in the inner regions where the multiple images are located and is therefore tightly constrained by the observations. The disc is sub-maximal and contributes 45 ± 11 per cent of the rotational support of the galaxy at 2.2rd. The halo mass is (2.0± 0.6)× 10 12M⊙, and the stellar to virial mass ratio is 7.0± 2.3 per cent, consistent with typical galaxies of the same mass.