Internal kinematics of spiral galaxies - Gas and stellar rotation curves and dispersion profiles

Emission- and absorption-line observations of six moderately inclined spiral galaxies are presented. The mean rotation and velocity dispersion for the gas and the stars have been measured. Two component mass models are fitted to surface brightness measurements are used to fit kinematic models to the velocity data. Models with constant mass-to-light ratios and isotropic velocity dispersions roughly fit the inner regions of spiral galaxies with three caveats: (1) several galaxies show significant kinematic differences between the two sides of the galaxy. While the differences might be caused by nonaxisymmetric potentials, variations in the line-of-sight extinction are a more likely cause; (2) the inner portion (R less than of equal to 1 kpc) of the emission-line (gas) rotation curve falls below the predicted circular velocity. Because the model circular velocities are computed from the observed stellar velocities, variations in the total mass-to-light ratio will not change the model values. It is noted that gas lost from bulge giants (planetary nebulae) will have a significant noncircular velocity component. It is shown that these emitted clouds live a sufficient time to explain the lower-than-circular velocity of the emission-line rotation curves; Some of the bulges are flatter than one would expect from their rotation rate, assuming isotropic random motions, but this additional flattening could be caused by the disk potential.