In recent years significant progress has been made in obtaining information about the physical conditions of the gaseous matter in the halo of the Milky Way. Progress has also been achieved in obtaining the beginning of a theoretical understanding of what ionizes and what supports gas found at large distances from the galactic plane. Theorists have begun to realize the important role the gaseous halo may play in providing a place for the gas in the underlying disk of the Milky Way to vent its energy.
The relationship between the gas of the Milky Way halo and the narrow absorption lines seen in the spectra of QSOs is only weakly established. While many QSO observers generally believe that a subset of the QSO absorption line phenomena likely is related to the absorption produced in the halos and/or extended disks of intervening galaxies as first proposed by Bahcall and Spitzer (1969), it is clear that an enormous amount of work remains to establish this connection with certainty and to determine what fraction of the observed features actually have their origin in gaseous halos.
Fortunately the Milky Way gaseous halo can be studied in many different ways (see Table 1) and a large body of observational and theoretical work has begun to provide important new insights about its properties. Since we know so little about the gaseous halos surrounding other galaxies, it seems appropriate to use the gaseous halo around our own galaxy as a basis for comparison with some of the phenomena seen in QSO spectra which might represent absorption by the halos and/or disks of other intervening spiral galaxies. It is not the intention of this review to emphasize the relation between what we see in our own halo and what is commonly seen in QSO spectra. Instead the emphasis will be on a discussion of what is currently known about the Milky Way's gaseous halo.
Many observers of galactic interstellar matter consider halo gas to be that gas having a distance away from the galactic plane, |z|, which exceeds 0.5 kpc. Furthermore many of the galactic studies of this gas are for matter at large |z| but in the solar region of the galaxy. In contrast, the QSO absorption line observers will most often only sample galaxies through absorption sight lines which pass through the outermost edges of intervening galaxies. Such sight lines may pass through disk gas and halo gas. Understanding the modifications caused by this major difference in sampling represents a very important first step in interrelating what we see locally in our own halo to the absorption we might expect to see at high redshift in the spectra of QSOs.
| Technique | Species or Gas Phase Sampled | References* |
| 21 cm emission | HI | 1, 2, 3 |
| optical absorption | CaII, TiII, NaI, KI | 4, 5, 6 |
| ultraviolet absorption | many ions | 7, 8, 9, 10 |
| optical emission | H , OIII
| 11 |
| ultraviolet emission | CIV, OIII | 12 |
| pulsar dispersion | e's | 13 |
| free-free absorption and emission | e's | 14 |
| X-ray emission | hot gas (106 °K) | 15, 16 |
| non-thermal radio emission | high energy e's and B field | 17 |
| Faraday rotation | e's and B field | 18 |
 Ray emission
| high energy e's and photon field | 19 |
*References (This list identifies recent reviews and/or research papers which provide a survey of the relevant literature). 1. Lockman (1984), 2. Lockman (1986), 3. Giovanelli (1986), 4. Albert (1983), 5. Jenkins (1986), 6. Morton and Blades (1986), 7. Savage and deBoer (1981), 8. Savage and Massa (1987), 9. Pettini and West (1982), 10. Savage and Jeske (1980), 11. Reynolds (1986), 12. Martin and Bowyer (1987), 13. Harding and Harding (1982), 14. Spitzer (1978 and references therein), 15. Marshall and Clark (1984), 16. Nousek et al. (1982), 17. Beuermann, Kanbach and Berkhuijsen (1985), 18. Sofue, Fujimoto and Wielebinski (1986), 19. Stecker (1979) | ||