The distribution and kinematics of neutral gas in M31

The kinematic signature of individual spiral arm segments are used here to make a very compact yet quite complete parameterization of the global neutral hydrogen content of M31. Two continuous, trailing spiral arms can be traced from about 0.5 to 27 kpc from the nucleus which accurately account for the kinematic and spatial distribution of gas. The three-dimensional geometry of the gas distribution is derived with only limited assumptions concerning axisymmetry of the velocity field. Significant variations in inclination as a function of azimuth and radius are found, leading to departures of the local midplane from a few hundred pc in a tilted disk in the inner galaxy to in excess of a kpc in the 'warp' of the outer galaxy. The distribution of exponential scale height of the gas with radius varies between 200 and 600 pc, and the spiral pattern in the inner galaxy has an elliptical distortion which leads to significant corrections to the axisymmetric rotation velocity and implied mass distribution. 48 refs.