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In this paper, we use the outer-galactic HI scale height data as well as theobserved rotation curve as constraints to determine the halo densitydistribution of the Andromeda galaxy (M31). We model the galaxy as agravitationally-coupled system of stars and gas, responding to the externalforce-field of a known Hernquist bulge and the dark matter halo, the densityprofile of the latter being characterized by four free parameters. Theparameter space of the halo is optimized so as to match the observed HIthickness 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 thebest fit to the observed data. This gives a central density of 0.011 M_sun/pc^3, a core radius of 21 kpc, and an axis ratio of 0.4. The main result fromthis work is the flattened dark matter halo for M31, which is required to matchthe outer galactic HI scale height data. Interestingly, such flattened haloslie at the most oblate end of the distribution of halo shapes found in recentcosmological simulations.

The Flattened Dark Matter Halo of M31 as Deduced from the Observed HI Scale Heights