| | 17 | The concept of warm and cold nuclear medium as equilibrium phases of the ISM were introduced in the paper by [http://adsabs.harvard.edu/abs/1969ApJ...155L.149F Field, Goldsmith, and Habing (1969)]. Afterward, an additional phase was introduced into the mix, the hot ionized medium by [http://adsabs.harvard.edu/abs/1977ApJ...218..148M 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 [http://adsabs.harvard.edu/abs/1995ApJ...455..536P Vazquez-Semadeni, Passot, Pouquet (1994)] point out. |
