INTERSTELLAR MEDIUM, GALACTIC CORONA BLAIR D. SAVAGE The gaseous galactic corona refers to the low density gaseous region extending away from the dense gas of the disk of the Milky Way galaxy into the halo for distances estimated to be at least 3000 pc. The corona is known to contain hot (-200,000 K), warm (-5000 K), and cool gas (-200 K). The galactic corona may be the result of violent explosive processes occurring in the Milky Way that heat and overpressurize the interstellar gas of the disk and cause it to flow away from the galactic plane into the halo, where it cools and rains down onto the galactic disk in a flow resembling that of a fountain. GAS OF THE GALACTIC DISK The gas in the galactic halo probably originates from the matter in the galactic disk. The astronomical observing advances brought by the space age have provided a much clearer picture of the nature of the gaseous medium found in the disk. The disk gas exists in a wide range of physical conditions. The gas temperatures range from about 30 K to 10* K and the densities range from about 10** to 10** atoms cm-*. The cold gas is mostly confined to a very thin plane of the Milky Way in a region about 200 pc thick. It is in this region of the Galaxy that the process of star formation is responsible for the conversion of dense clouds of interstellar gas into stars of different masses. The direct evidence for the existence of the hot phase of the interstellar medium in the galactic disk has come from measurements with space instrumentation of the x-rays that the gas produces and of the ultraviolet absorption toward distant stars produced by highly ionized atoms such as O+* and N+*. The x-rays provide direct diagnostic information on the existence of 10* K gas with densities of about 10** atoms cm**, whereas the ultraviolet absorption measures of highly ionized atoms reveal the existence of somewhat cooler gas. The hot gas of the galactic disk may be the fundamental substrate in which all the other gas phases are found. However, currently there is considerable uncertainty concerning the actual fraction of interstellar space filled by the hot phase. PROPERTIES OF THE GAS OF THE GALACTIC CORONA The existence of the hot gas in the galactic disk opens the possibility that gas may also be found at large distances away from the plane in the halo. The degree of confinement of gas to the plane of a galaxy depends on the temperature of the gas and the strength of the gravitational attraction of the matter in the disk. For a constant gravitational acceleration * toward the disk, gas at a uniform temperature T will assume an exponential density distribution away from the plane of the galaxy given by ************************, where n is the matter density in atoms per cubic centimeter, z is the distance from the plane of the galaxy, H=kT/mg is the scale height of the gas with Boltzmann's constant k=1.38x10** erg K-* and m is the average particle mass in grams. For ionized gas with a temperature T=10* K in the solar region of the Milky Way the scale height of the gas is about 6000 pc. Therefore, gas as hot as 10* K in the disk of the Milky Way will tend to flow outward from the plane of the Galaxy into the halo region. During the 1980s the evidence slowly accumulated that gas does indeed exist at large distances from the plane of the Milky Way. The evidence came from a variety of diagnostic techniques. Measures of the 21-cm radio line emission from neutral hydrogen in interstellar space revealed the existence of large expanding loop-like structures, known as supershells, with extents of up to 2000 pc. These structures probably represent the hydrogen at the swept-up boundaries of regions of gas set into motion by the collective effects of many supernova explosions that occur in groups of young massive stars known as OB associations. The large sizes of the structures imply the ejection of gas into the galactic halo. Measures of the propagation of the pulses of radio radiation from rapidly rotating neutron stars (pulsars) revealed that the free electrons of interstellar space exist in a layer having an exponential scale height of 1000 pc from the galactic plane. At ultraviolet wavelengths, the spectrographs aboard the International Ultraviolet Explorer satellite have been used to study the distribution of highly ionized gas in the interstellar medium with the result that atoms of N+*,C+*, and Si+* were found to exist in a very thick layer with a scale height of about 3000 pc. The gaseous region that extends away from the galactic plane to distances of 500-3000 pc or more is commonly referred to as the gaseous galactic halo or corona. The gas of the corona has a wide range of temperatures. The highest temperature so far measured is approximately 200,000 K, which is the temperature required to produce substantial quantities of the N+* found in halo gas. However, gas with temperatures ranging all the way down to less than 200 K is also found and it is likely that gas with temperatures greater than 200,000 K also exist. The medium appears to be highly inhomogeneous in its conditions with there being hot low density regions and cooler clouds of higher density. The composition of the halo gas is not well determined, but the available evidence points toward a gas phase composition similar to that found in the atmosphere of the Sun, with most of the matter being hydrogen and helium with trace amounts of the heavier elements. In halo gas most of the heavy elements appear to be in the gas phase rather than in the solid phase. This is in contrast to the composition of the gas found in the galactic disk where observations reveal a marked reduction in the gas phase abundance of many heavy elements compared to the solar composition. This reduction or depletion is interpreted as being the result of many of the heavy elements, such as silicon, iron, and magnesium, being present in the solid particles of interstellar dust. Apparently in the halo gas, the amount of dust present is greatly reduced and therefore most of a given heavy element is found in the gas phase rather than the solid phase. Gas found at large distances from the galactic plane exhibits a complex pattern of motions. In addition to the existence of relatively quiescent regions such as found for the gaseous matter seen toward the south galactic pole, there exist disturbed regions with gas moving up away from the galactic plane and also toward the plane. Extensive rapidly moving clouds of hydrogen, known as high velocity clouds, have been found in the Galaxy. The pattern of motion exhibited by these clouds is consistent with the idea that they represent the return of matter from the halo to the disk of the Milky Way. ORIGN OF THE GASEOUS GALACTIC CORONA In recent years the theoretical understanding of the new observational data on gas at large distances from the plane of the Galaxy has developed into a theory known as the galactic fountain model. In this theory the great explosions of supernovae create hot over-pressurized regions of gas that can burst out of the plane of the Galaxy and allow the flow of hot highly ionized gas into the halo. The outflowing gas proceeds to cool and can then rain back down onto the galactic plane as cooler gas clouds in a flow resembling that of a fountain of water. The outflowing gas has temperatures of perhaps 10* K or higher. Such gas has proven difficult to detect. However, gas cooling in the fountain can explain the observed amount of N+* and C+* seen at large distances from the galactic plane. In the flow associated with a galactic fountain, one would expect to detect the motions of parcels of cooler gas with speeds of up to 150 km s**. Such motions might provide an explanation for the high velocity hydrogen clouds observed by radio astronomers. In groups of massive stars known as OB associations, many supernovae may occur over time scales of about 10* years. Such temporally correlated explosions will create superbubbles of hot overpressurized gas that can rapidly expand and break out of the plane of the Galaxy, forming structures called "chimneys." This process is similar to that of the galactic fountain, but the ejection of gas is highly concentrated in the chimneys rather than over the entire disk of the Galaxy. Perhaps the actual situation is a combination of a widespread fountain activity with localized very active regions where chimneys are found. Not all theorists of the interstellar medium agree with the basic ideas of the fountain model or of the chimney model. In particular, some astronomers believe that there is a more quiescent interstellar medium in which the gas found at large distances from the galactic plane is supported by the pressure of the magnetic field and cosmic rays of the Galaxy. In such a cosmic-ray-supported galactic halo, the highly ionized gas might have its origin through the process of photoionization where the ionizing radiation might come from extragalactic space or from hot stars in the halo. Although such models can explain the existence of Si+* and C+* found at large distances from the galactic plane, they have difficulties accounting for the observed amount of N+*. Theories advocating this more quiescent picture of the interstellar medium might apply in some regions of the Galaxy, whereas the dynamic phenomena associated with fountains, superbubbles, and chimneys might apply elsewhere. Studies of the gaseous corona of the Milky Way have so far provided information about the matter in the solar region of the Milky Way. However, there are indications that in the very central regions of the Galaxy the gas at large distances from the plane may behave differently than in the solar region. For distances from the galactic center of less than about 3 Kpc there is much less hydrogen away from the galactic plane than found elsewhere. The central region of the Milky Way may be a place where the supernova activity is so great that instead of having a bound circulating corona as in the galactic fountain model, the gas leaves the Galaxy in the form of a wind. There is evidence for enhanced x-ray emission from the general direction of the central region of the Galaxy. That emission may be associated with hot outflowing gas situated above and below the center of the Milky Way. In many respects the gaseous galactic corona bears a similarity to the hot coronae of the Sun and other stars. The solar corona contains gas at many different temperatures ranging from 5000 to 2x10* K. Disturbances in lower layers of the Sun spew gas into the solar corona. The ejected gas has been seen to move along lines of magnetic field, to condense into cooler matter, and to form giant prominences that then flow back to the solar surface layer. The processes that heat and thereby maintain the solar corona are believed to be various types of mechanical disturbances that arise in the turbulent surface region known as the photosphere. The disturbances propagate outward in the form of various types of mechanical waves, deposit their energy, and heat the overlying gas. Many clues about what may be happening on the galactic scale might be obtained from a study of those processes occurring in the outer atmosphere of the Sun. GASEOUS CORONAS OF OTHER GALAXIES Galaxies other than the Milky Way also have extended gaseous halos or coronas. The overall similarities or differences between the coronas of other galaxies and that of the Milky Way will depend critically on the frequency of occurrence of violent stellar processes that are able to create zones of hot overpressurized gas in the disks of those galaxies. A very active galaxy might be capable of converting much of its gas into hot gas and driving a galactic wind and producing a very extensive but short lived corona. In contrast, a galaxy with a low supernova rate will hardly produce any hot gas and such a galaxy would not be likely to have a dynamic gaseous corona like that proposed for the Milky Way. The gaseous coronas of spiral galaxies do not appear to be hot enough and dense enough to be bright x-ray emitters. Data currently available regarding the possible existence of gaseous halos around external spiral galaxies mostly come from radio and optical astronomical studies. For certain galaxies viewed edge on, it has been found that neutral and ionized hydrogen sometimes occur in both confined and extended components. The extended components may represent the halo gas. In the case of the nearby Andromeda galaxy, which is a spiral galaxy similar to the Milky Way, our viewing position is from above its disk. Radio studies of Andromeda have revealed the existence of holes in the neutral hydrogen in the disk gas with extents of 100-1000 pc. Those holes may represent the regions where multiple supernova explosions have created hot cavities of gas with the subsequent venting of gas into the halo. The phenomena occurring in the Milky Way that are responsible for the ejection of gas into the halo may be occurring in many other galaxies in the universe as well. Additional Reading de Boer, K.S. and Savage, B.D.(1982). The coronas of galaxies. Scientific American 247 (No. 2) 54. Savage, B.D.(1988). The properties of the gaseous galactic corona. In QSO Absorption Lines: Probing the Universe, C. Blades, C. Norman, and D. Turnshek, eds. Cambridge University Press, Cambridge, p. 195. Spitzer, L.(1990). Theories of the hot interstellar gas. Ann. Rev. Astron. Ap. 28 71. York, D.G.(1982). Gas in the galactic halo. Ann. Rev. Astron. Ap. 20 221.