The radial variation of HI 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 (HI) to study HI velocity dispersion and HI phases as a
function of galaxy morphology in 22 galaxies from The HI Nearby Galaxy Survey
(THINGS). We stack individual HI velocity profiles and decompose them into
broad and narrow Gaussian components. We study the HI velocity dispersion and
the HI surface density, 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 HI velocity dispersion 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 HI and warm HI, tends to decrease with radius. On average, this
ratio is lower in dwarfs than in spirals. This lack of a narrow, cold HI
component in dwarfs may explain their low star formation activity.
