  We present a method to derive the dynamical mass of face-on galaxy disks
using their neutral hydrogen (HI) velocity dispersion. We have applied the
method to nearby, gas rich galaxies that have extended HI gas disks and have
low inclinations. The galaxy sample includes 4 large disk galaxies; NGC628,
NGC6496, NGC3184, NGC4214 and 3 dwarf galaxies DDO46, DDO63 and DDO187. We have
used archival data from the THINGS and LITTLE THINGS surveys to derive the HI
gas distributions and SPITZER mid-infrared images to determine the stellar disk
mass distributions. We examine the disk dynamical and baryonic mass ratios in
the extreme outer disks where there is HI gas but no visible stellar disk. We
find that for the large galaxies the disk dynamical and Hi gas mass surface
densities are comparable in the outer disks. But in the smaller dwarf galaxies,
for which the total HI gas mass dominates the baryonic mass i.e.
M(HI)>M(stars), the disk dynamical mass is much larger than the baryonic mass.
For these galaxies there must either be a very low luminosity stellar disk
which provides the vertical support for the HI gas disk or there is halo dark
matter associated with their disks, which is possible if the halo has an oblate
shape so that the inner part of the dark matter halo is concentrated around the
disk. Our results are important for explaining the equilibrium of HI disks in
the absence of stellar disks, and is especially important for gas rich, dwarf
galaxies that appear to have significant dark matter masses associated with
their disks.
