H I and CO velocity dispersions in nearby galaxies

We analyze the velocity dispersions of individual H i and CO profiles in a number of nearby galaxies from the high-resolution HERACLES CO and THINGS H i surveys. Focusing on regions with bright CO emission, we find a CO dispersion value ${\sigma }_{CO}$ = 7.3 ± 1.7 km s⁻¹. The corresponding H i dispersion ${\sigma }_{HI}$ = 11.7 ± 2.3 km s⁻¹, yielding a mean dispersion ratio ${\sigma }_{HI}/{\sigma }_{CO}$ = 1.4 ± 0.2, independent of radius. We find that the CO velocity dispersion increases toward 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 ratio ${\sigma }_{HI}/{\sigma }_{CO}$ = 1.0 ± 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.