A wide-field HI mosaic of Messier 31. II.The disk warp, rotation and the
  dark matter halo
  We test cosmological models of structure formation using the rotation curve
of the nearest spiral galaxy, M31, determined using a recent deep, full-disk
21-cm imaging survey smoothed to 466 pc resolution. We fit a tilted ring model
to the HI data from 8 to 37 kpc. The disk of M31 warps from 25 kpc outwards and
becomes more inclined with respect to our line of sight. Newtonian dynamics
without a dark matter halo provide a very poor fit to the rotation curve
derived using the warp model. In the framework of modified Newtonian dynamic
however the 21-cm rotation curve is well fitted by the gravitational potential
traced by the baryonic matter density alone. The inclusion of a dark matter
halo with a density profile as predicted by structure formation in a
hierarchical clustering LambdaCDM cosmology makes the mass model in newtonian
dynamic compatible with the rotation curve data. The dark halo concentration
for the best fit is C=12 and its total mass is 1.2 10^{12} Msun. If a dark halo
model with a constant density core is considered, the core radius has to be
larger than 20 kpc in order for the model to fit to the data. We extrapolate
the best-fit LambdaCDM and constant-density core mass models to very large
galactocentric radii, comparable to the size of the dark matter halo. A
comparison of the predicted mass with the M31 mass determined at such large
radii using other dynamical tracers, confirms the validity of our results. In
particular the LambdaCDM dark halo model which best fits the 21-cm data well
reproduces the M31 mass traced out to 560 kpc. Our estimated total mass of M31
is 1.3 10^{12} Msun, with 12% baryonic fraction and only 6% of the baryons in
neutral gas.
