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.