2.3. Examples of thermal phases
Field, Goldsmith and Habing (FGH: 1969) produced the first specific model for a two-phase equilibrium of the interstellar medium (ISM), in which radiative cooling is balanced by cosmic ray heating. The two phases in the FGH model include cold clouds (T ~ 100 K) and a warm intercloud medium (T ~ 104 K). Other heating mechanisms which may be important (probably more important than cosmic rays [Spitzer 1978]) include diffuse UV and X-ray flux, photoelectric emission by normal grains (Draine 1978; de Jong 1980; Shull and Woods 1985) or polycyclic aromatic hydrocarbons (PAHs: d'Hendecourt and Leger 1987; Lepp and Dalgarno 1988), mechanical heating (Cox 1979), magnetoacoustic waves (Spitzer 1982; Ikeuchi and Spitzer 1984), and ion-neutral friction (Scalo 1977; Ferrière, Zweibel and Shull 1988). The characteristic temperatures of the warm and cold thermal phases are insensitive to the details of the heating processes; they simply reflect the energies of the resonance and fine-structure lines, respectively, responsible for cooling the gas. Gas at ~ 104 K may exist in a range of ionization states, and McKee and Ostriker (1977) drew a distinction between the "warm neutral medium" and a "warm (photo)ionized medium" irradiated by UV from hot stars. A molecular phase at ~ 10 K is now known to contain most of the mass in the ISM of the Milky Way, but this component appears to form self-gravitating clouds which are out of pressure balance with the rest of the ISM. A phase diagram for these phases is computed by Lepp et al. (1985).
The gas which emits the broad emission lines in AGN has also been modeled as part of a stable two-phase medium (McCray 1979; Krolik, McKee, and Tarter 1981 [KMT]; Lepp et al. 1985; Krolik 1988). On the basis of observations, the line-emitting gas is inferred to be concentrated in many small clouds which fill a tiny fraction of the volume of the emission line region (Davidson 1972). Compton heating by the observed X-rays provides the minimum level of heating of the hot component of the medium; additional heating due to relativistic particles, radio frequency heating, cloud friction, and shocks may also be important (KMT). Cooling of the hot phase be homogeneous and hot or in two phases; and finally, there is usually a range of densities for which the gas must be in two phases (cf. Fig. 2 and Section 2.2). KMT showed that unless the temperature of the hot gas in the broad line region is well above 108 K, most of the mass is in the hot phase, corresponding to the hot/two-phase case.
Cox and Smith (1974) pointed out that the cooling time of interstellar gas shock-heated by supernova remnants could be longer than the interval between the passage of successive shocks. This suggestion led to the three-phase model of the ISM (McKee and Ostriker 1977), in which most of the volume is occupied by shock-heated gas. This ~ 106 K gas is an example of a non-equilibrium phase, the possibility of which was foreseen by Spitzer (1956). Because it is produced dynamically, and has a temperature of order the virial temperature of the Galaxy, it has proven very difficult to determine the fate of the hot intercloud medium. It is not at all clear whether it cools radiatively in a region close to the disk (McKee and Ostriker 1977) or is vented into the halo through "chimneys" (McCray and Kafatos 1987; Norman and Ikeuchi 1989), where it undergoes a combination of adiabatic and radiative cooling (the "Galactic fountain": Shapiro and Field 1976; Cox 1981; Wang and Cowie 1988). It is also not known whether the hot gas cools sufficiently in the halo to form clouds which eventually rain down on the disk, remains hot enough to drive a galactic wind, or somehow does both. Finally, the effects of spatial correlations among Type II supernovae (in OB associations) are just beginning to be appreciated (McCray and Kafatos 1987).
Cooling flows in elliptical galaxies and galaxy clusters are also thought to have a nonequilibrium two-phase structure. When the existence of cooling flows was first recognized (Cowie and Binney 1977; Fabian and Nulsen 1977), it was pointed out that the cooling gas should be thermally unstable to the formation of cool (~ 104 K) filaments (Fabian and Nulsen 1977; Mathews and Bregman 1978; Cowie, Fabian and Nulsen 1980). Optical emission lines have been observed in the central regions of many cooling flows (Lynds 1970; Heckman 1981; Cowie et al. 1983; Hu, Cowie and Wang 1985; Johnstone, Fabian and Nulsen 1987; Heckman et al. 1989). However, the development of linear thermal instability is severely hampered by buoyancy (Balbus 1988; Balbus and Soker 1989), and it is not clear whether the filaments grow from finite but small perturbations or are advected inward in a highly nonlinear form (Nulsen 1986). Furthermore, the mechanism which excites the emission lines is very uncertain, and may play a role creating and maintaining the multiphase structure. Multiphase models of cooling flows have been studied by Nulsen (1986); Thomas, Fabian and Nulsen (1987); Thomas (1988); and Böhringer and Fabian (1989).
An extreme version of the cooling flow instability has been proposed to account for the masses of protogalaxies (Rees and Ostriker 1977; Silk 1977) and of globular clusters (Fall and Rees 1985). The basic idea of these models is that a self-gravitating gas cloud will fragment only when its cooling time becomes shorter than its free-fall time, and then it will develop a two-phase structure in which just enough material drops out of the hot phase to keep the cooling time roughly comparable to the free-fall time. Characteristic mass scales are determined by the Jeans mass of the cold phase in pressure balance with the hot phase. Triggering of star formation by radio lobes expanding into a protogalactic multiphase medium has been proposed (Rees 1989; Begelman and Cioffi 1989) to account for the observed radio/optical alignments in high-redshift radio galaxies (McCarthy et al. 1987; Chambers, Miley and van Breugel 1987). Cool gas in the multiphase protogalactic environment might give rise to some quasar absorption line systems (Hogan 1987) as well as the extended emission-line "fuzz" around high-redshift quasars (Rees 1988).