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The degeneracy among the disc, bulge and halo contributions to galaxy rotation curves prevents an understanding of the distribution of baryons and dark matter in disc galaxies. In an attempt to break this degeneracy, we present an analysis of the strong gravitational lens SDSS J2141−0001, discovered as part of the Sloan Lens ACS survey. The lens galaxy is a high‐inclination, disc‐dominated system. We present new  Hubble Space Telescope  multicolour imaging, gas and stellar kinematics data derived from long‐slit spectroscopy and  K ‐band laser guide star adaptive optics imaging, both from the Keck telescopes. We model the galaxy as a sum of concentric axisymmetric bulge, disc 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 (disc plus bulge) stellar mass of log 10 ( M  * /M ⊙ ) = 10.99 +0.11  − 0.25 . The photometric data combined with stellar population synthesis models yield log 10 ( M  * /M ⊙ ) = 10.97 ± 0.07 and 11.21 ± 0.07 for the Chabrier and Salpeter initial mass functions (IMFs), respectively. Assuming no cold gas, a Salpeter IMF is marginally disfavoured, with a Bayes factor of 2.7. Accounting for the expected gas fraction of ≃ 20 per cent reduces the lensing plus kinematics stellar mass by 0.10 ± 0.05 dex, resulting in a Bayes factor of 11.9 in favour of a Chabrier IMF. The dark matter halo is roughly spherical, with minor to major axis ratio  q  3, h = 0.91 +0.15  − 0.13 . The dark matter halo has a maximum circular velocity of  V  max = 276 +17  − 18  km s −1 , and a central density parameter of log 10 Δ  V /2 = 5.9 +0.9  − 0.5 . This is higher than predicted for uncontracted dark matter haloes in Λ cold dark matter cosmologies, log 10 Δ  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. Larger samples of spiral galaxy strong gravitational lenses are needed in order to distinguish between these two possibilities. At 2.2 disc scalelengths the dark matter fraction is  f  DM = 0.55 +0.20  − 0.15 , suggesting that SDSS J2141−0001 is submaximal.

The SWELLS survey – II. Breaking the disc–halo degeneracy in the spiral galaxy gravitational lens SDSS J2141−0001 ★