A spiral galaxy's mass distribution uncovered through lensing and
  dynamics
  We investigate the matter distribution of a spiral galaxy with a
counter-rotating stellar core, SDSS J1331+3628 (J1331), independently with
gravitational lensing and stellar dynamical modelling. By fitting a
gravitational potential model to a quadruplet of lensing images around J1331's
bulge, we tightly constrain the mass inside the Einstein radius R_ein = (0.91
+/- 0.02)'' (~= 1.83 +/- 0.04 kpc) to within 4%: M_ein = (7.8 +/- 0.3) x 10^10
M_Sun. We model observed long-slit major axis stellar kinematics in J1331's
central regions by finding Multi-Gaussian Expansion (MGE) models for the
stellar and dark matter distribution that solve the axisymmetric Jeans
equations. The lens and dynamical model are independently derived, but in very
good agreement with each other around ~R_ein. We find that J1331's center
requires a steep total mass-to-light ratio gradient. A dynamical model
including an NFW halo (with virial velocity v_200 ~= 240 +/- 40 km/s and
concentration c_200 ~= 8 +/- 2) and moderate tangential velocity anisotropy
(beta_z ~= -0.4 +/- 0.1) can reproduce the signatures of J1331's
counter-rotating core and predict the stellar and gas rotation curve at larger
radii. However, our models do not agree with the observed velocity dispersion
at large radii. We speculate that the reason could be a non-trivial change in
structure and kinematics due to a possible merger event in J1331's recent past.
