H I Kinematics and Dynamics of Messier 31

We present a new deep 21 cm survey of the Andromeda galaxy, based on high-resolution observations performed with the Synthesis Telescope and the 26 m antenna at the Dominion Radio Astrophysical Observatory. The H I distribution and kinematics of the disk are analyzed and basic dynamical properties are given. The rotation curve is measured out to 38 kpc, showing a nuclear peak at 340 km s^-1, a dip at 202 km s^-1 around 4 kpc, two distinct flat parts at 264 km s^-1 and 230 km s^-1, and an increase to 275 km s^-1 in the outermost regions. Except for the innermost regions, the axisymmetry of the gas rotation is very good. A very strong warp of the H I disk is evidenced. The central regions appear less inclined than the average disk inclination of 74°, while the outer regions appear more inclined. Mass distribution models by ΛCDM Navarro-Frenk-White, Einasto or pseudo-isothermal dark matter halos with baryonic components are presented. They fail to reproduce the exact shape of the rotation curve. No significant differences are measured between the various shapes of halo. The dynamical mass of M31 enclosed within a radius of 38 kpc is (4.7 ± 0.5) × 10¹¹ M _sun. The dark matter component is almost four times more massive than the baryonic mass inside this radius. A total mass of 1.0 × 10¹² M _sun is derived inside the virial radius. The spectral structure of the observations is very complex, with sometimes up to five H I components detected per spectrum, which is very rarely seen in other galaxies. New H I structures are discovered in the datacube. The most remarkable new features are an external arm and thin H I spurs in the disk outskirts. A relationship between these spurs and outer stellar clumps is evidenced. The external arm is 32 kpc long, lies on the far side of the galaxy, and has no obvious counterpart on the other side of the galaxy. Its kinematics clearly differs from the outer adjacent disk. Both these H I perturbations could result from tidal interactions with galaxy companions like NGC 205.

Observations obtained at the Dominion Radio Astrophysical Observatory (DRAO), operated as a national facility by the National Research Council of Canada.