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 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 \sim { 1.5 } \arcsec ( \sim 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 \gamma =0.9 \pm 0.3 for \rho _ { DM } \propto r ^ { - \gamma } , a bulge with M/L _ { B } =6.6 \pm 0.3 \Upsilon _ { \odot } , and a disc with M/L _ { B } =1.2 \pm 0.3 \Upsilon _ { \odot } , 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 \pm 11 per cent of the rotational support of the galaxy at 2.2r _ { d } .
The halo mass is ( 2.0 \pm 0.6 ) \times 10 ^ { 12 } M _ { \odot } , and the stellar to virial mass ratio is 7.0 \pm 2.3 per cent , consistent with typical galaxies of the same mass .