Face-on galaxies

A 21 cm survey of about 200 face-on galaxies using the 305 m Arecibo radio telescope is reported. The narrowest profiles for every morphological type are presented. The data are consistent with an integral velocity dispersion of about 8 km/s in every morphological type. When combined with the results of other studies, the present results indicate that the gas dispersion appears to be a universal number, broadly independent of morphological type, absolute magnitude, and position within a galaxy. Two direct consequences follow from the universality of an isotropic velocity distribution: (1) the 'missing mass' component within galaxies necessarily resides in the halo, and (2) small galaxies may only maintain an H I distribution through galaxy-wide random motions, provided that the appropriate dispersion is the canonical one.