DISENTANGLING BARYONS AND DARK MATTER IN THE SPIRAL GRAVITATIONAL LENS B1933+503

Measuring the relative mass contributions of luminous and dark matter in spiral galaxies is important for understanding their formation and evolution. The combination of a galaxy rotation curve and strong lensing is a powerful way to break the disk-halo degeneracy that is inherent in each of the methods individually. We present an analysis of the 10 image radio spiral lens B1933+503 at z l = 0.755, incorporating (1) new global very long baseline interferometry observations, (2) new adaptive-optics-assisted K-band imaging, and (3) new spectroscopic observations for the lens galaxy rotation curve and the source redshift. We construct a three-dimensionally axisymmetric mass distribution with three components: an exponential profile for the disk, a point mass for the bulge, and a Navarro-Frenk-White (NFW) profile for the halo. The mass model is simultaneously fitted to the kinematics and the lensing data. The NFW halo needs to be oblate with a flattening of a/c = 0.33 +0.07 -0.05 to be consistent with the radio data. This suggests that baryons are effective at making the halos oblate near the center. The lensing and kinematics analysis probe the inner 10 kpc of the galaxy, and we obtain a lower limit on the halo scale radius of 16 kpc (95% credible intervals). The dark matter mass fraction inside a sphere with a radius of 2.2 disk scale lengths is f DM,2.2 = 0.43 +0.10 -0.09 . The contribution of the disk to the total circular velocity at 2.2 disk scale lengths is 0.76 +0.05 -0.06 , suggesting that the disk is marginally submaximal. The stellar mass of the disk from our modeling is log 10 (M * /M ) = 11.06 +0.09 -0.11 assuming that the cold gas contributes 20% to the total disk mass. In comparison to the stellar masses estimated from stellar population synthesis models, the stellar initial mass function of Chabrier is preferred to that of Salpeter by a probability factor of 7.2.