HI and CO Velocity Dispersions in Nearby Galaxies
  We analyze the velocity dispersions of individual HI and CO profiles in a
number of nearby galaxies from the high-resolution HERACLES CO and THINGS HI
surveys. Focusing on regions with bright CO emission, we find a CO dispersion
value: 7.3 $\pm$ 1.7 km/s. The corresponding HI dispersion is 11.7 $\pm$ 2.3
km/s, yielding a mean HI/CO dispersion ratio of 1.4 $\pm$ 0.2, independent of
radius. We find that the CO velocity dispersion increases towards lower peak
fluxes. This is consistent with previous work where we showed that when using
spectra averaged ("stacked") over large areas, larger values for the CO
dispersion are found, and a lower dispersion ratio: 1.0 $\pm$ 0.2. The stacking
method is more sensitive to low-level diffuse emission, whereas individual
profiles trace narrow-line, GMC-dominated, bright emission. These results
provide further evidence that disk galaxies contain not only a thin, low
velocity dispersion, high density CO disk that is dominated by GMCs, but also a
fainter, higher dispersion, diffuse disk component.
