Distribution and kinematics of neutral hydrogen in the spiral galaxy M81. II. Analysis.

Summary. In this paper we analyze the data presented in Paper I. From the velocity field we derive a rotation several geometrical parameters: Beyond 10 kpc the velocity -field is asymmetric, possjbly due - fQ tidal interaction wjth the- companion-.galaxies. A mass - -on the -curve, and - the light. distribution obtained from the data- by Brandt et al. (1972) - yield M/L ratios between 1.5 and- -8 (corrected for internal absorption). H/M and H/L in9rease. with radius - out - to about 10 kpc,-while.B . The of H I shows a regular two- armed spiral pattern which can be -approximated by a logarithmic spiral -with a pitch angle of 150. There is a close with the optical spiral structure. The peak-to-average contrast is estimated to be 1.7 in the inner part of the eastern arm and in the entire western arm; in the outer parts of the eastern arm it increases to 2.3. The eastern arm seems in many respects to be a perfect undisturbed spiral arm; whereas the - western.arm is much fuzzier. - From the .velocity and jhe rotatidn curve a residual - velocity field is which bonfain only - the line-of-sight cbmponents of.all -non- velocities. This velocity cdmpared with-similar fields ..predicted by. linear density-wave-Theory models. Given the limitations of linear density wave theory, a reasdnable agreement can be achieved by assuming a phase shift of 200 betwe.en the H I spiral arms and the pote.ntiai .minimum-. In. our models we adopt a pattern speed of 20 km/s/kpc, which puts corotation at 10.3 kpc. Key words: neutral hydrogen - spiral galaxies - Messier 81 mass distribution spiral structure