THE MOLECULAR FRONT IN GALAXIES .2. 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 t he literatures. We demonstrate that the gas phase transition of HI and H-2 occurs suddenly within a narrow range of radius, which we call th e molecular front. We have tried to explain such phase transition in g alactic scale with a help of the phase transition theory proposed by E lmegreen. The crucial parameters for determinating the molecular fract ion f(mol) are interstellar pressure P, UV radiation field U, and meta llicity 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 galaxi es which has an exponential disk, and that the metallicity gradient is most crucial for the formation of the molecular front. We have also t ried to reproduce the observed molecular fraction f(mol) by giving the set of (P, U, Z) observationally, and show that the model can describ e the variation of the molecular fraction f(mol) in galaxies quite wel l. We discuss the implication of the molecular front for the chemical evolution of galaxies.