Stars and dark matter in the spiral gravitational lens 2237+0305

We construct a mass model for the spiral lens galaxy 2237+0305, at redshift z_l = 0.04, based on gravitational-lensing constraints, HI rotation, and new stellar-kinematic information, based on data taken with the Echelle Spectrograph and Imager (ESI) spectrograph on the 10-m 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 ~ 1.5arcsec (~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 ρ_DM ~ r^-γ, a bulge with M/L_B = 6.6 +/- 0.3Υ_solar, and a disc with M/L_B = 1.2 +/- 0.3Υ_solar, 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 submaximal and contributes 45 +/- 11 per cent of the rotational support of the galaxy at 2.2r_d. The halo mass is (2.0 +/- 0.6) × 10¹²M_solar, and the stellar to virial mass ratio is 7.0 +/- 2.3 per cent, consistent with typical galaxies of the same mass.