The Molecular Front in Galaxies II : Galactic-scale Gas Phase Transition of HI and H2
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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.
Latex, 16 pages, Figures on request, To appear in AA
Latex, 16 pages, Figures on request, To appear in AA