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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%). It is dominate at low pressures.
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. It is dominate at high pressures.
The concept of warm and cold nuclear medium as equilibrium phases of the ISM were introduced in the paper by Field, Goldsmith, and Habing (1969). The concept is motivated by terrestrial water at sea level. At that equilibrium pressure, a phase transition occurs between water liquid and water vapor. It is though that HI behaves similarly.
Afterward, an additional phase was introduced into the mix, the hot ionized medium by McKee and Ostriker (1977). There are other components in the ISM such as dust, cosmic rays, for example. The exact concentration, filling fractions, and dynamic vs. equilibrium state of the material is not well understood, however, as Vazquez-Semadeni, Passot, Pouquet (1994) point out.
