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.
