4.1 The Composition of the ISM
The composition of the ISM is dominated by hydrogen which contributes 90% of all interstellar matter in either its molecular, neutral or ionized form. Based on Spitzer (1978) one can produce the following breakdown for the composition of the ISM:
Averaged over the Galaxy, about equal amounts of molecular and neutral hydrogen are found; each component comprises ~ 4 × 109 M. The ratio of molecular to atomic gas varies as a function of position, the molecular gas being concentrated in giant molecular cloud complexes which are most abundant in the molecular ring. There is a steep decrease of molecular material as a function of radius and atomic hydrogen dominates outside the solar circle. Hydrogen in its ionized form has until recently been solely associated with HII regions, contributing about 1% in mass. However, as argued by Kulkarni and Heiles (1987) there now is ample evidence that diffuse ionized hydrogen is ubiquitous in the solar neighborhood and that there might be about 109 M of it present in the Galaxy.
Helium is usually considered to be uniformly mixed with hydrogen. It makes up ~ 8% of the ISM by number, but contributes ~ 40% by mass. This point is often overlooked, especially by radio astronomers when they quote the mass of neutral gas in a galaxy based on observations of the 21-cm line of HI. Other elements, such as C, N, O, Ne and Fe make up only minor fractions of at most 10-3 to 10-4 by number. All other atoms and molecules can be considered as trace elements. This, of course, does not in any sense reduce their tremendous value for probing the physical conditions of the ISM. Dust particles and grains are thought to contribute 1-2% of the mass of a typical interstellar environment. Lastly, the ISM is permeated by a magnetic field of a few µG which constrains the motion of cosmic ray particles, mainly protons.
4.2 The Structure and Phases of the ISM
The various components which make up the ISM in the Galaxy can find themselves in any of the following five phases (Mihalas and Binney 1981, Kulkarni and Heiles 1988):
As mentioned above, the molecular material is mostly confined to molecular clouds which are held together by gravitation. The cold, warm and hot phases are in global pressure equilibrium. The filling factor for each of the phases is highly uncertain, as is the topology of the ISM (see also section 4.3). The values listed above represent what are currently thought to be the most realistic estimates. It is in trying to determine these values that observations of nearby galaxies will be most useful.
In general, the ISM has structures on all scale lengths, from smaller than 1 pc to larger than 1000 pc. HI observations of the Galaxy have painted a picture of a violent ISM. Heiles (1979, 1984) has shown that there exist dozens of HI shells and supershells, some of them showing clear signs of expansion. The energetics involved suggest that the HI shells are the result of the cumulative effect of supernovae going off within their parent OB association. The shells are the swept up matter whereas the inner portions of these shells are probably filled with HIM. This picture of the ISM is confirmed by HI observations of M31 (Brinks and Bajaja 1986) and other nearby galaxies which will be described in more detail below. The features which Heiles calls supershells are too energetic to be explained in this way (see for a thorough discussion the review article by Tenorio-Tagle and Bodenheimer 1988) and it might be necessary to invoke a different mechanism, such as the infall of material from outside the galaxy.
4.3 Models Describing the ISM
Several models have been proposed over the years to explain the ever more detailed observations of the ISM. The first attempt to explain the different absorption and emission characteristics of the ISM was due to Clark (1965) whose description was formalized by Field, Goldsmith and Habing (1969) in their famous two-phase model for the ISM. In modern terms, they describe the ISM as consisting of a cold and a warm neutral phase in static equilibrium. Cox and Smith (1974) soon realized that the ISM is far from static and that supernovae dominate the energy input. They noticed that the cooling time of the HIM created by SN explosions is comparatively long and that the supernova blown bubbles can link up to form a tunneling network.
In order to better understand Cox and Smith's model and subsequent descriptions of the ISM it is perhaps useful to review briefly the evolution of a region of massive star formation or OB association. The most massive stars, i.e., M > 25 M, firstly ionize their immediate surroundings and blow an initial cavity though the action of their stellar winds. Each star quickly evolves and explodes as a supernova after some 5 × 106 year, depositing about 1051 erg in the ISM, partly in the form of kinetic energy, enlarging the wind blown cavity. Less massive stars with M > 8 M evolve more slowly and will go off as supernovae after about 5 × 107 year. Although the most massive stars have a large influence on their surroundings via photoionization and stellar winds, averaged over a typical galaxy, the larger number of less massive stars makes that the effects of supernova explosions dominate (assuming a normal Initial Mass Function or IMF, of course). An instructive calculation of the energy balance of the local ISM has been given by Abbott (1982). A more complete account is given by Tenorio-Tagle and Bodenheimer (1988) who review the multitude of models which describe the evolution of an ensemble of O and B stars and the effects which they have on the surrounding ISM.
McKee and Ostriker (MO, 1977) incorporated the idea put forward by Cox and Smith into their famous thee phase model, in which the cool, warm, and hot phases of the ISM are in global pressure equilibrium. The MO model proved a major step forward and has been very successful in explaining several key features of the ISM, e.g., the occurrence of highly ionized lines such as the OVI line which was observed in absorption by the Copernicus satellite, the interstellar pressure, and the observed cloud dispersion. However, partly as a result of increased observational efforts over the last decade, it has come under serious criticism (cf. Shull 1987). Some of the main shortcomings of the MO model are:
These findings have prompted considerable efforts to try to improve upon the MO model and to gain a better understanding of the ISM. Ikeuchi et al. (1984) propose a set of models based on MO which incorporate time evolutionary effects. They show that depending on the supernova rate and ambient density the ISM can either be in a three phase or a two phase state, or even cycle between two states. Heiles (1987) explicitly takes into account the difference in distribution between Type I and Type II supernovae and uses a more reasonable value for the supernova rate. In a more recent effort Heiles (1989) reviews the observational evidence related to the number of supernovae per OB association and takes into account the effect which the thickness of the HI layer has on the confinement of expanding HI shells. He includes in his calculations the full thickness of the disk, including the low density part which decreases exponentially with increasing distance from the plane, known as the Lockman disk (Lockman 1984).
A possible way to avoid creating models which have too high a filling factor for the HIM is to vent hot material into the halo via a galactic fountain (Bregman 1980, Cox 1981, Corbelli and Salpeter 1988). A particularly interesting model for the ISM in this respect is the one proposed by Norman and Ikeuchi (1989) in which they combine the virtues of the MO model with those of the galactic fountain models. Norman and Ikeuchi show that in their chimney model they can allow for a two phase and a thee phase regime à la MO by simply changing the SN rate. In their model most of the hot phase resides in the halo of the Galaxy.
It will be clear that the main problem, in fact, does not lie with a lack of models and ideas, but with the fundamental limitations posed by our unfavorable position within the disk of the Galaxy. In order to test the validity of the improved descriptions for the ISM it will be necessary to look at nearby galaxies in as much detail as possible.