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Gas velocity dispersions provide important diagnostics of the forcescounteracting gravity to prevent collapse of the gas. We use the 21 cm line ofneutral atomic hydrogen (HI) to study HI velocity dispersion and HI phases as afunction of galaxy morphology in 22 galaxies from The HI Nearby Galaxy Survey(THINGS). We stack individual HI velocity profiles and decompose them intobroad and narrow Gaussian components. We study the HI velocity dispersion andthe HI surface density, as a function of radius. For spirals, the velocitydispersions of the narrow and broad components decline with radius and theirradial profiles are well described by an exponential function. For dwarfs,however, the profiles are much flatter. The single Gaussian dispersion profilesare, in general, flatter than those of the narrow and broad components. In mostcases, the dispersion profiles in the outer disks do not drop as fast as thestar formation profiles, derived in the literature. This indicates theimportance of other energy sources in driving HI velocity dispersion in theouter disks. The radial surface density profiles of spirals and dwarfs aresimilar. The surface density profiles of the narrow component decline moresteeply than those of the broad component, but not as steep as what was foundpreviously for the molecular component. As a consequence, the surface densityratio between the narrow and broad components, an estimate of the mass ratiobetween cold HI and warm HI, tends to decrease with radius. On average, thisratio is lower in dwarfs than in spirals. This lack of a narrow, cold HIcomponent in dwarfs may explain their low star formation activity.

The radial variation of HI velocity dispersions in dwarfs and spirals