We present an updated mass model for M31 that makes use of a Spitzer 3.6 \mu m 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 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 that is well outside the range predicted in \Lambda CDM cosmology and is therefore disfavoured .
An adiabatically contracted NFW halo favors an initial concentration c _ { vir } = 20 and virial mass 8.2 \times 10 ^ { 11 } M _ { \odot } , and this is in line with the cosmological expectations for a galaxy of the size of M31 .
The best-fit 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 can not be deduced from imaging data due to the galaxy ’ s inclination ) is P = 24 \aas@@fstack { \circ } 7 \pm 4 \aas@@fstack { \circ } 4 .