THE DISTRIBUTION AND KINEMATICS OF NEUTRAL GAS IN M31

The kinematic signature of spiral arm segments in the highly inclined gaseous disk of M31 is identified within previously published data bases of neutral hydrogen emission. These features are tabulated to give a complete spatial and kinematic parameterization of the neutral gas distribution. Neither a fixed inclination nor a fully axisymmetric velocity field were assumed in the analysis. Significant variations in the local orientation are derived, including a systematic departure of the inner (2 kpc) disk to an orientation tilted by 15-degrees with respect to the outer disk. The spatial distribution of gas is very well described by a global two-arm trailing spiral. The spiral in the inner 5 kpc has an elliptical distortion with an axial ratio of about 1.9 which is coaligned with the long axis of the apparently triaxial stellar bulge. Between 5.4 and 16.3 kpc the spiral pattern is logarithmic with pitch angle 6.7-degrees. Beyond 17 kpc the two spiral arms merge into a single structure. The rotation curve, after correction for the inner ellipticity, is well fitted with just two mass components: a stellar bulge and disk with mass distributions which are traced by the blue light. A total mass M = (2.0 +/- 0.1) x 10(11) M. is found within 28 kpc. Ratios of mass to blue luminosity for both the bulge and disk are 6.5 M./L.. There is no evidence for additional mass with a more uniform spatial distribution out to 28 kpc. Identifying the two boundaries of the range in radius over which the spiral is logarithmic with (1) the inner Lindblad resonance and (2) corotation leads to a pattern speed of 15 +/- 2 km s-1 kpc-1 and a predicted outer Lindblad resonance at 22 kpc, where the two arms are observed to merge. The most obvious driver of the spiral pattern seems to be M32, which may also be responsible for many of the departures from a planar distribution as well as significant asymmetries in rotation velocity between 12 and 16 kpc radius. The radial dependence of gas scale height is found to be roughly linear, extending from about 200 pc at 5 kpc radius to about 600 pc at 25 kpc.