Theories for the origin of the gaseous galactic halo must be able to explain the support and ionization of the gas. Two competing models for the support of the gas are the `galactic fountain model' (Shapiro and Field 1976; Bregman 1980; also see Chevalier and Oegerle 1979 and Habe and Ikeuchi 1980) and the cosmic ray supported halo models (Hartquist, Pettini and Tallant 1984; Chevalier and Fransson 1984). In the galactic fountain model it is noted that gas will flow into the halo as a consequence of supernova explosions which heat and elevate the pressure of gaseous regions in the galactic disk. The regions of elevated pressure may breakout from the galactic plane and provide an injection of gas into the halo where the gas may cool and return to the disk in a flow pattern resembling a fountain. Calculations of the hydrodynamics of the breakout process produced by correlated supernovae in OB associations are found in Mac Low and McCray (1988) while the physical effects of falling clouds on the galactic disk are considered by Tenorico-Tagle et al. (1987). A process that will occur on a more gradual basis is `galactic convection'. Here it is recognized from far ultraviolet absorption line studies of OVI (Jenkins 1978) and soft diffuse X-ray studies (McCammon et al. 1983) that there exists in the galactic disk a hot (T 0.3 to 2 x 106 °K) low density (n 10-3 cm-3) phase of the interstellar medium which may fill much of the volume of the galactic disk. Hot (106 °K) gas in the solar region of the galaxy is buoyant. Its thermal scale height is about 7 kpc. The gas will therefore tend to flow outward away from the galactic plane into the halo where it may cool and return to the disk as cooler clouds in a convective like flow. The spatial characteristics observed for the cool gas as traced through its HI 21 cm emission illustrated in Figure 3 is suggestive of a fountain like or convective like flow. In the cosmic ray supported halo models it is assumed that the gas is supported at its large z distances by the pressure of outwardly streaming cosmic rays.
For the ionization of the gas, the possibilities for the production of the highly ionized species (SiIV , CIV and NV) include electron collisional ionization in 0.8 to 3 x 105 °K gas and photoionization by hot white dwarf stars (Dupree and Raymond 1983), by normal Population I stars, and by the extragalactic EUV background. A number of recent calculations have concentrated on determining the production of the highly ionized gas by photoionization (Hartquist, Pettini and Tallant 1984; Chevalier and Fransson 1984; Fransson and Chevalier 1985; Bregman and Harrington 1986). The predictions of Chevalier and Fransson (1984) are shown in Figure 5d. From this work it appears possible to understand the observed amounts of SiIV and CIV and in particular the sudden rise in N(ion)|sin b| near z = 1 kpc from photoionization by the EUV background. However, the various calculations have difficulty producing the observed amount of NV (Savage and Massa 1985, 1987). NV is an important ion since among those ions accessible to the IUE, it requires the greatest amount of energy for its production (77 eV). Most hot stars containing He have strong He+ edges at 54 eV. Therefore, the only stellar sources that might be capable of converting NIV into NV are the very hot hydrogen white dwarfs. In order to explain the observed NV, Edgar and Chevalier (1986) calculated the amount of SiIV, CIV, NV and OVI produced in cooling gas in a galactic fountain flow. For a galactic fountain mass flow rate of 4 M yr-1 on each side of the galactic plane, they predict the column densities perpendicular to the galactic plane listed in Table 3. We see that in the cooling fountain flow enough NV is produced to explain the observations. Their calculation also predicts CIV and OIII] emission line strengths which are compatible with the recent diffuse background measurements of Martin and Bowyer (1987). The theory predicts a large OVI column density [N(OVI) = 5.8-6.0 x 1014 cm-2] while the measurements of Jenkins (1978) yield a column density of approximately 3 x 1013 cm-2 out to a |z| of about 1 kpc. If the theory is correct such a large difference will require the presence of substantial quantities of OVI in the halo for |z| > 1 kpc. Future measures with the proposed Lyman spacecraft will provide a crucial test of the cooling fountain gas theory.
|Ion||Predicted a N(cm-2)||Observed b N(cm-2)|
|SiIV||(3.3-6.4) x 1012||2 x 1013|
|CIV||(4.3-7.9) x 1013||1 x 1014|
|NV||(2.8-3.6) x 1013||3 x 1013|
|OVI||(5.8-6.0) x 1014||> 3 x 1013|
aPredicted values assume a fountain mass flow rate of 4 M yr-1 to each side of the galactic plane. The values listed are column densities through the halo (N Sin |b|) on one side of the galaxy.
Savage and Massa (1987) found that the explanation for the support and ionization of halo gas requires a blending of the ideas from the galactic fountain models and the photoionized halo models. In this situation SiIV would mostly be produced by photoionization while NV would mostly be produced by collisional ionization in cooling fountain gas. CIV therefore, probably represents an intermediate situation with important contributions from collisional ionization and photoionization.
If a galactic fountain flow actually exists, it seems reasonable to interpret the inflowing neutral gas at large |b| as gas associated with that flow. This interpretation is consistent with the abundance studies of Caldwell (1979) for HD93521 and Albert (1983) for TiII toward many stars. Hot (106 °K) upflowing disk gas would be expected to be devoid of grains because of rapid destruction during the heating of that gas. When this hot gas cools and returns to the plane, it is unlikely that conditions will be favorable for grain formation. Therefore, the returning gas should have nearly solar abundances in the gas phase as is observed. In a galactic fountain flow the relative amounts of gas residing in high ionization states (e.g. SiIV, CIV and NV) versus gas in low ionization states (e.g. SiII, CII and NII) will be influenced by the cooling rates of the gas through the various temperature regimes. The observation that the column density of weakly ionized gas exceeds that of the highly ionized gas by about a factor of ten seems compatible with the fact that gas rapidly cools from about 3 x 105 to 104 °K.
The gaseous galactic halo is a region of our galaxy that is now ideally suited for innovative observational and/or theoretical studies. In a brief period of time (about 10 years) the subject of the gaseous halo has moved from one where even the very existence of such a region was doubted by many to one which is now considered of vital importance for the overall regulation of the interstellar medium of the galactic disk (e.g. see Cox 1981 and Cowie 1987).