Vertical equilibrium of molecular gas in galaxies

We present CO(1-0) and CO(2-1) observations of the two nearly face-on galaxies NGC 628 and NGC 3938, in particular cuts along the major and minor axis. The contribution of the beam-smeared in-plane velocity gradients to the observed velocity width is quite small in the outer parts of the galaxies. This allows us to derive the velocity dispersion of the molecular gas perpendicular to the plane. We find that this dispersion is remarkably constant with radius, 6 \kms for NGC 628 and 8.5 \kms for NGC 3938, and of the same order as the \hI\ dispersion. The constancy of the value is interpreted in terms of a feedback mechanism involving gravitational instabilities and gas dissipation. The similarity of the CO and \hI dispersions suggests that the two components are well mixed, and are only two different phases of the same kinematical gas component. The gas can be transformed from the atomic phase to the molecular phase and vice-versa several times during a z-oscillation.