A revised Λ CDM mass model for the Andromeda Galaxy

We present an updated mass model for M31 that makes use of a Spitzer 3.6-μm image, a mass-to-light ratio gradient based on the galaxy's B - R colour profile, and observed rotation curve data from a variety of sources. We examine cases where the dark matter follows a pure Navarro, Frenk & White (NFW) profile and where an initial NFW halo contracts adiabatically in response to the formation of the galaxy. We find that both of these scenarios can produce a reasonable fit to the observed rotation curve data. However, a pure NFW model requires a concentration c_vir = 51 which is well outside the range predicted in Λcold dark matter cosmology and is therefore disfavoured. An adiabatically contracted NFW halo favours an initial concentration c _vir = 20 and virial mass 8.2 × 10¹¹ M _⊙, and this is in line with the cosmological expectations for a galaxy of the size of M31. The best-fitting mass is consistent with published estimates from Andromeda Stream kinematics, satellite galaxy radial velocities and planetary nebulae studies. Finally, using the known linear correlation between rotation curve shear and spiral arm pitch angle, we show that the stellar spiral arm pitch angle of M31 (which cannot be deduced from imaging data due to the galaxy's inclination) is P=24°7 ± 4°4.