Gross Properties of Five Scd Galaxies as Determined from 21-CENTIMETER Observations

Neutral-hydrogen studies of the five Scd galaxies M33, NGC 2403, IC 342, MIOl, and NGC 6946 have revealed substantial similarities in their hydrogen distributions and rotation curves. We find that 80 percent of the observable II 1 lies within the boundary defined by the Holmberg diameter for each galaxy. The ratio of the H 1 mass to the total mass contained within this same boundary has a mean value of 0.11. The central H 1 surface density is also nearly constant within the Scd classification. We find that all the H 1 global properties and ratios are nearly constant or scale approximately with Holmberg radius or total mass. An observed correlation of maximal rotation velocity with Holmberg radius allows characterization of the total mass with only one parameter. Because of the very flat rotation curves observed for these Scd galaxies, total masses extrapolated to infinite radius are not known. Surface mass-luminosity ratios must reach values of 20 at the HoImberg radius.