THE RADIAL VARIATION OF H I VELOCITY DISPERSIONS IN DWARFS AND SPIRALS

Gas velocity dispersions provide important diagnostics of the forces counteracting gravity to prevent collapse of the gas. We use the 21 cm line of neutral atomic hydrogen (H I) to study H I velocity dispersion (sigma(H I)) and H I phases as a function of galaxy morphology in 22 galaxies from The H I Nearby Galaxy Survey. We stack individual H I velocity profiles and decompose them into broad and narrow Gaussian components. We study the H I velocity dispersion and the H I surface density, Sigma(H I), as a function of radius. For spirals, the velocity dispersions of the narrow and broad components decline with radius and their radial profiles are well described by an exponential function. For dwarfs, however, the profiles are much flatter. The single Gaussian dispersion profiles are, in general, flatter than those of the narrow and broad components. In most cases, the dispersion profiles in the outer disks do not drop as fast as the star formation profiles derived in the literature. This indicates the importance of other energy sources in driving sigma(H I) in the outer disks. The radial surface density profiles of spirals and dwarfs are similar. The surface density profiles of the narrow component decline more steeply than those of the broad component, but not as steep as what was found previously for the molecular component. As a consequence, the surface density ratio between the narrow and broad components, an estimate of the mass ratio between cold H I and warm H I, tends to decrease with radius. On average, this ratio is lower in dwarfs than in spirals. This lack of a narrow, cold H I component in dwarfs may explain their low star formation activity.