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 VLBI observations, (2) new
adaptive-optics assisted K-band imaging, (3) new spectroscopic observations for
the lens galaxy rotation curve and the source redshift. We construct a
three-dimensionally axisymmetric mass distribution with 3 components: an
exponential profile for the disk, a point mass for the bulge, and an 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% CI). 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_{sun}) = 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.
