The SWELLS survey. II. Breaking the disk-halo degeneracy in the spiral
  galaxy gravitational lens SDSS J2141-0001
  The degeneracy among the disk, bulge and halo contributions to galaxy
rotation curves prevents an understanding of the distribution of baryons and
dark matter in disk galaxies. In an attempt to break this degeneracy, we
present an analysis of the spiral galaxy strong gravitational lens SDSS
J2141-0001, discovered as part of the SLACS survey. We present new Hubble Space
Telescope multicolor imaging, gas and stellar kinematics data derived from
long-slit spectroscopy, and K-band LGS adaptive optics imaging, both from the
Keck telescopes. We model the galaxy as a sum of concentric axisymmetric bulge,
disk and halo components and infer the contribution of each component, using
information from gravitational lensing and gas kinematics. This analysis yields
a best-fitting total (disk plus bulge) stellar mass of log_{10}(Mstar/Msun) =
10.99(+0.11,-0.25). The photometric data combined with stellar population
synthesis models yield log_{10}(Mstar/Msun) = 10.97\pm0.07, and 11.21\pm0.07
for the Chabrier and Salpeter IMFs, respectively. Accounting for the expected
gas fraction of \simeq 20% reduces the lensing plus kinematics stellar mass by
0.10\pm0.05 dex, resulting in a Bayes factor of 11.9 in favor of a Chabrier
IMF. The dark matter halo is roughly spherical, with minor to major axis ratio
q_{halo}=0.91(+0.15,-0.13). The dark matter halo has a maximum circular
velocity of V_{max}=276(+17,-18) km/s, and a central density parameter of
log_{10}\Delta_{V/2}=5.9(+0.9,-0.5). This is higher than predicted for
uncontracted dark matter haloes in LCDM cosmologies, log_{10}\Delta_{V/2}=5.2,
suggesting that either the halo has contracted in response to galaxy formation,
or that the halo has a higher than average concentration. At 2.2 disk scale
lengths the dark matter fraction is f_{DM}=0.55(+0.20,-0.15), suggesting that
SDSS J2141-0001 is sub-maximal.
