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We present a comprehensive study of the velocity dispersion of the atomic(HI) and molecular (H2) gas components in the disks (R < R25) of a sample of 12nearby spiral galaxies with moderate inclinations. Our analysis is based onsensitive high resolution data from the THINGS (atomic gas) and HERACLES(molecular gas) surveys. To obtain reliable measurements of the velocitydispersion, we stack regions several kilo-parsecs in size, after accounting forintrinsic velocity shifts due to galactic rotation and large-scale motions. Westack using various parameters: the galacto-centric distance, star formationrate surface density, HI surface density, H2 surface density, and total gassurface density. We fit single Gaussian components to the stacked spectra andmeasure median velocity dispersions for HI of 11.9 +/- 3.1 km/s and for H2 of12.0 +/- 3.9 km/s. The CO velocity dispersions are thus, surprisingly, verysimilar to the corresponding ones of HI, with an average ratio ofsigma(HI)/sigma(CO) = 1.0 +/- 0.2 irrespective of the stacking parameter. Themeasured CO velocity dispersions are significantly higher (factor 2) than thetraditional picture of a cold molecular gas disk associated with starformation. The high dispersion implies an additional thick molecular gas disk(possibly as thick as the HI disk). Our finding is in agreement with recentsensitive measurements in individual edge-on and face-on galaxies and pointstowards the general existence of a thick disk of molecular gas, in addition tothe well-known thin disk in nearby spiral galaxies.

A high-dispersion molecular gas component in nearby galaxies