The Molecular Front in Galaxies II : Galactic-scale Gas Phase Transition of HI and H2

We have examined the distribution of HI and H$_2$ gases in four face-on galaxies by using the observed dat a 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 radi us, 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 $f_{\rm mol}$ are interstellar pressure $P $, UV radiation field $U$, and metallicity $Z$, and we have constructed a model galaxy in which $P$, $U$ a nd $Z$ 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 exponentia l 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 $f_{\rm mol}$ by giving the set of ($P$, $ U$, $Z$) observationally, and show that the model can describe the variation of the molecular fraction $f_ {\rm mol}$ in galaxies quite well. We discuss the implication of the molecular front for the chemical evolution of galaxies.