H i Survey of Face-on Galaxies: The Frequency of Distortions in H i Disks

The full results of an H I survey of face-on galaxies are presented and it is shown that narrow H I profiles are rare in normal spiral galaxies. This is due in part to the wider-than-expected range of the integral dispersion and in part to the frequent occurrence of large-scale distortions in the H I disk. These factors reduce the number of galaxies with half-power widths less than 30 km/s to about 24 percent of those that would occur if galaxies generally had quiescent, coplanar H I disks. Two useful subsets may be drawn from this study of 212 face-on galaxies with axial ratios greater than 0.87. Fifty-two spirals of all morphological types have half-power widths smaller than 100 km/s and may be used for studies that benefit from a small velocity spread and an enhanced beam-filling factor. About 40 galaxies have velocity widths much larger than expected and are of interest in studies of dynamically peculiar systems.