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Evolution of Molecular and Atomic Gas Phases in the Milky Way

We analyze radial and azimuthal variations of the phase balance between the molecular and atomic ISM in the Milky Way. In particular, the azimuthal variations -- between spiral arm and interarm regions -- are analyzed without any explicit definition of spiral arm locations. We show that the molecular gas mass fraction, i.e., fmol=H2/ (HI+H2) in mass, varies predominantly in the radial direction: starting from ~100% at the center, remaining ~>50% (~>60%) to R~6kpc, and decreasing to ~10-20% (~50%) at R=8.5 kpc when averaged over the whole disk thickness (in the mid plane). Azimuthal, arm-interarm variations are secondary: only ~20%, in the globally molecule-dominated inner MW, but becoming larger, ~40-50%, in the atom-dominated outskirts. This suggests that in the inner MW, the gas stays highly molecular (fmol>50%) as it goes from an interarm region, into a spiral arm, and back into the next interarm region. Stellar feedback does not dissociate molecules much, and the coagulation and fragmentation of molecular clouds dominate the evolution of the ISM at these radii. The trend differs in the outskirts, where the gas phase is globally atomic (fmol<50%). The HI and H2 phases cycle through spiral arm passage there. These different regimes of ISM evolution are also seen in external galaxies (e.g., LMC, M33, and M51). We explain the radial gradient of fmol by a simple flow continuity model. The effects of spiral arms on this analysis are illustrated in Appendix.