1995A&A...296...33S

We have obtained composite position-velocity (PV) diagrams of the HI and CO line emissions for edge-on galaxies NGC 891, NGC 4565, NGC 5907, and NGC 3079. Using a de-convolving method of the PV diagram, we have obtained radial distributions of HI and H₂ gas densities, and the radial variation of the molecular fraction as defined by the mass ratio of the molecular (H₂) to total (HI+H₂) gas density has been calculated. Radial variations of the molecular fraction shows that the galaxy disk is almost totally molecular in the central few kpc region. However, the molecular fraction decreases suddenly in a narrow range of radius, beyond which the interstellar gas is almost totally HI, and we call this critical radius the `molecular front'. We point out that the molecular and atomic gases in disk galaxies are clearly separated by this front, which will be the place where the phase transition from HI to H₂, and vice versa, is taking place in the galactic scale. We discuss the implication of the molecular-front phenomenon for the evolution of interstellar gas in galaxies based on the phase-transition theory of the ISM.