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 \pm 1.7 km s ^ { -1 } .
The corresponding H i dispersion \sigma _ { HI } = 11.7 \pm 2.3 km s ^ { -1 } , yielding a mean dispersion ratio \sigma _ { HI } / \sigma _ { CO } = 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 ratio \sigma _ { HI } / \sigma _ { CO } = 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 .