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H2 vs. HI vs. HII
H2 - molecular hydrogen
HI - neutral atomic hydrogen
HII - ionized atomic hydrogren
Phases of the ISM
Molecular clouds - Traced by CO, held together by their own self-gravity rather than being in pressure equilibrium with ambient. Typically about 10 K, 40 pc across, 200 particles/cm3, 400,000 solar masses. Mostly made up of H2, but this is hard to see in transition lines and is instead inferred from CO detection. MCs make up ~ 1% of the filling fraction of the ISM, but contains about 30-60% of the mass.
Warm nuclear medium - Traced by 21 cm line of hydrogen (emitted by neutral hydrogen atom), ~800 K, is intercloud gas. Has a very low density (~0.5 / cm3), but a substantial filling fraction (30-60%).
Cold nuclear medium - Also traced by 21 cm line of hydrogen. Is about 100 K and organized in diffuse HI clouds. Occupies about 1-4% the total volume, and on average a cloud is about 10 pc across and has a density of 50 / cm3.
