Precision velocity fields in spiral galaxies. II. Rotation curves

Imaging Fabry-Perot data has been acquired for a sample of seven late-type spiral galaxies from which two-dimensional velocity fields have been constructed on a subkiloparsec resolution scale. The kinematic disk (i.e., tilted ring) modeling procedure is used to fit the velocity field and recover its rotational structure. From this fit, different schemes are used to extract the major-axis rotation curve, as well as velocity profiles along the minor axis. Rotation curves found from deprojection of the velocity field using mean geometric parameters invariably produce small-scale "bumps and wiggles," which are a reflection of the inclusion of noncircular motions. In the best-rotation curve method, the rotation curve is derived from the best fit to the entire velocity field. This method is shown to minimize the effects of noncircular motions. The kinematic center of the velocity field is allowed to vary, and those results are directly compared with cases where the kinematic center is held fixed. In virtually all cases, we find that the variable center model produces the smoothest and most symmetric rotation curves and minor-axis velocity profiles. Our results indicate that the kinematic center of mass is moving around in these disk systems with an amplitude of plus or minus similar to 300 pc. Finally, we investigate the effects of small, systematic errors in centering and find that small minor-axis displacements can artificially produce spurious slopes in the rotation curve at large radius, including producing rotation curves that are apparently falling.