The flattened dark matter halo of M31 as deduced from the observed H I scale heights

Abstract

In this paper we use the outer galactic H I scale height data as well as the observed rotation curve as constraints to determine the halo density distribution of the Andromeda galaxy (M31). We model the galaxy as a gravitationally coupled system of stars and gas, responding to the external force field of a known Hernquist bulge and the dark matter halo, the density profile of the latter being characterized by four free parameters. The parameter space of the halo is optimized so as to match the observed H I thickness distribution as well as the rotation curve on an equal footing, unlike the previous studies of M31 which were based on rotation curves alone. We show that an oblate halo, with an isothermal density profile, provides the best fit to the observed data. This gives a central density of 0.011 M_⊙ pc^-3, a core radius of 21 kpc, and an axis ratio of 0.4. The main result from this work is the flattened dark matter halo for M31, which is required to match the outer galactic H I scale height data. Interestingly, such flattened halos lie at the most oblate end of the distribution of halo shapes found in recent cosmological simulations.