The molecular front in galaxies. II. Galactic-scale gas phase transition of HI and H_2_.

We have examined the distribution of HI and H_2_ gases in four face-on galaxies by using the observed data of CO and HI line emissions from the literatures. We demonstrate that the gas phase transition of HI and H_2_ occurs suddenly within a narrow range of radius, which we call the molecular front. We have tried to explain such phase transition in galactic scale with a help of the phase transition theory proposed by Elmegreen. The crucial parameters for determinating the molecular fraction f_mol_ are interstellar pressure P, UV radiation field U, and metallicity Z, and we have constructed a model galaxy in which P, U and Z obey an exponential function of the galacto-centric radius. The model shows that the molecular front must be a fundamental feature of galaxies which has an exponential disk, and that the metallicity gradient is most crucial for the formation of the molecular front. We have also tried to reproduce the observed molecular fraction f_mol_ by giving the set of (P, U, Z) observationally, and show that the model can describe the variation of the molecular fraction f_mol_ in galaxies quite well. We discuss the implication of the molecular front for the chemical evolution of galaxies.