Three-dimensional distribution of the ISM in the Milky Way Galaxy. IV. 3D molecular fraction and Galactic-scale HI-to-H2 transition

Abstract

Three-dimensional (3D) distribution of the volume-density molecular fraction, defined by fρmol = ρH2/(ρHI + ρH2 ), is studied in the Milky Way Galaxy. The molecular front appears at galacto-centric distance of R 8 kpc, where the galactic-scale phase transition from atomic to molecular hydrogen occurs with fρmol dropping from 0.8 to 0.2 within a radial interval as narrow as 0.5 kpc. The fρmol front is much sharper than that of the surface density molecular fraction. The fρmol front also appears in the direction vertical to the galactic plane with a full width of the high- fρmol disk to be 100 pc. The radial and vertical fρmol profiles, particularly the front behavior, are fitted by theoretical curves calculated using the observed density profile and assumed radiation field andmetallicity with exponential gradients. The molecular fraction was found to be enhanced along spiral arms at radii R 6 to 10 kpc, such as the Perseus arm. This implies that the molecular clouds are produced from HI in the arms and are dissociated in the interarm regions in the transition region around the molecular front. We also show that there is a threshold value of mean HI density, over which HI is transformed into molecular gas.