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.
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