Precision velocity fields in spiral galaxies. I. Noncircular motions and RMS noise in disks

Investigation of the symmetry of the major- and minor-axis rotation curves reveals strong evidence of nonconcentric gas orbits with the maximum center shift of similar to 300 pc. Comparisons between kinematic and photometric structure (e.g., position angles, inclinations, centers) show considerable noise on small scales. Although large-scale averages are in agreement, this noise is a matter of some concern in the application of the Tully-Fisher method to disk galaxies. Moreover, cases of significant misalignment in position angle between the inner and outer disks are seen in two of the sample galaxies and may indicate the transition between luminous and dark-matter-dominated regions (i.e., where the maximum-disk hypothesis begins to fail). The kinematic disk models are used to find the residual velocity fields, and typical residuals are found to be 10-15 km s(-1) over regions 0.5-1.5 kpc in diameter. Correlations are shown to exist between the residual velocity fields and both the Ha intensity and the velocity dispersion images. This suggests that kinematic feedback to the gas from star formation is an important source of noncircular motion. However, the relative quiescence of the large-scale velocity held indicates that the effect does not cause a significant deviation from circular symmetry, kinematically indicating that star formation is not a hidden parameter in the Tully-Fisher relation. Finally, the residual velocity fields are examined for signs of noncircular orbits by looking for azimuthal angular harmonics that would be present if disk galaxies are embedded in a triaxial dark matter potential. For our sample we find the ellipticity of the gas orbits to be less than or similar to 0.08, which implies the potential is relatively round. This is consistent with disks being maximal.