标题： 银河系中分子和原子气体相的演化 显示英文标题

标题： Evolution of Molecular and Atomic Gas Phases in the Milky Way

摘要： 我们分析了银河系中分子和原子星际介质（ISM）相平衡的径向和方位变化。 特别是，方位变化——在旋臂和旋臂间区域之间——是在没有任何旋臂位置显式定义的情况下进行分析的。 我们表明，分子气体质量分数，即fmol=H2/(HI+H2)（按质量计算），主要在径向方向上变化：从中心的~100%开始，在R~6kpc处保持~>50%（~>60%），并在整个盘厚度（在中平面）平均后，在R=8.5 kpc处下降至~10-20%（~50%）。 方位上的旋臂-旋臂间变化是次要的：在整体分子占主导的内银河系中仅约20%，但在原子占主导的外围区域则变得更大，约为40-50%。 这表明在银河系内部，气体在从旋臂间区域进入旋臂，再进入下一个旋臂间区域时，仍保持高度分子态（fmol>50%）。 恒星反馈不会使分子大量解离，分子云的凝聚和碎片化在这些半径处主导了星际介质的演化。 在外围区域，趋势有所不同，那里的气体相总体上是原子态（fmol<50%）。 在那里的旋臂经过时，HI和H2相会循环。 这些不同的星际介质演化模式也在外部星系中被观察到（例如，大麦哲伦星云、M33和M51）。 我们通过一个简单的流动连续性模型解释fmol的径向梯度。 螺旋臂对该分析的影响在附录中进行了说明。 显示英文摘要

摘要： 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.