THE MOLECULAR FRONT IN GALAXIES .1. CO VS HI IN POSITION-VELOCITY DIAGRAMS

We have obtained composite position-velocity (PV) diagrams of the HI a nd 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 H-2 gas densities, and the ra dial variation of the molecular fraction as defined by the mass ratio of the molecular (H-2) to total (HI + H-2) gas density has been calcul ated. Radial variations of the molecular fraction shows that the galax y disk is almost totally molecular in the central few kpc region. Howe ver, the molecular fraction decreases suddenly in a narrow range of ra dius, beyond which the interstellar gas is almost totally HI, and we c all this critical radius the 'molecular front'. We point out that the molecular and atomic gases in disk galaxies are clearly separated by t his front, which will be the place where the phase transition from HI to H-2, and vice versa, is taking place in the galactic scale. We disc uss the implication of the molecular-front phenomenon for the evolutio n of interstellar gas in galaxies based on the phase-transition theory of the ISM.