Spiral structure and gas motion in M81

From the two-component spiral-structure model proposed by Marochnik, Mishurov, and Suchkov (1972), the spiral-arm geometry in the galaxy M81 is calculated and the density-wave parameters are evaluated: the pattern speed is approximately 20 km/sec kpc and the effective rotating component has a surface density of about 50 solar masses/sq pc. The nonlinear gas flow in the gravitational field of the arms is calculated for these parameters. Comparison with the observed radial distribution of H II regions, the radial-velocity profile of the gas along the minor axis, and the gas density profiles in the arms indicates that the spiral gravitational-force amplitude F is approximately 5 percent at 8-kpc distance from the center of the system.