A high resolution hydrogen line survey of Messier 31. II: The warped flaring hydrogen layer

Some of the global aspects of the H I distribution in M31 are discussed on the basis of the high-resolution survey of the 21-cm line made with the Westerbork Synthesis Radio Telescope by Brinks and Shane (1984). The morphology of the H I kinematics and structure in M31 can be modelled by a flat disk extending over the region at R < 16 kpc, and, at larger radii, a flaring warped gas distribution. The scale height of the gas in the warped part of M31 increases approximately linearly with radius to a maximum scale height of about 1.7 kpc. The maximum deviation of the warp from the plane of the extended inner disk is about 3.8 kpc. These and other parameters of the warp in M31 are similar to those in our Galaxy. The H I mass in the disk is about 2.4×10^9_M_sun;; the H I mass in the warp is about 1.5×10⁹M_sun;. As in our Galaxy, the flaring gas layer and the rather flat rotation curve in the regions of M31 require that the total mass there resides in a halo. The 2π-symmetry and the general regularity of the warp in M31 argue against it being a consequence of interaction with the small elliptical companion M32.