Tracing the Dynamical Mass in Galaxy Disks Using H i Velocity Dispersion and Its Implications for the Dark Matter Distribution in Galaxies

Abstract

We present a method to derive the dynamical mass of face-on galaxy disks using their neutral hydrogen (H i) velocity dispersion (σ H i ). We have applied the method to nearby, gas-rich galaxies that have extended H i gas disks and have low inclinations. The galaxy sample includes four large disk galaxies, NGC 628, NGC 6496, NGC 3184, and NGC 4214, and three dwarf galaxies, DDO 46, DDO 63, and DDO 187. We have used archival H i data from The H i Nearby Galaxy Survey (THINGS) and the LITTLE THINGS survey to derive the H i 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 H i gas but no visible stellar disk. We find that for the large galaxies, the disk dynamical and H i gas mass surface densities are comparable in the outer disks. But in the smaller dwarf galaxies, for which the total H i gas mass dominates the baryonic mass, i.e., M(H i) ≥ 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 H i 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 H i 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.