I would like to start by asking the question: "Why is this chapter in this book?" The emphasis of these lectures is clearly on stellar astrophysics, so why is it necessary to have a chapter dedicated to the interstellar medium?
The answer to that question lies in Antonio's invitation to me to participate in this winter school: "The school will be devoted to what things can be learned about galaxies from their stars, but properties of the gas are an important nexus between stars and galaxy evolution."
I couldn't agree more. An important long term goal of observational extragalactic astronomy is to construct detailed evolutionary histories of galaxies. This goal is best visualized using the "population box" concept introduced by Hodge (1989). Hodge produced schematic representations of star formation rates and chemical abundance evolution for many nearby galaxies (cf. Figure 18 in Gary DaCosta's chapter in this book). Converting these schematic representations into hard data (with error bars) represents a great challenge that all conference participants can fully appreciate. In the ensuing years since Hodge first published his review, there has been tremendous progress in the task, especially with regard to the Local Group dwarf ellipticals (much of this progress due to the capabilities of the Hubble Space Telescope). An excellent example of progress in adding detail to Hodge's diagrams can be found in the study of the dwarf irregular galaxy NGC 6822 by Gallart et al. (1996a, b, c).
I can add another reason for including a chapter on the interstellar medium (ISM). It is important to remind stellar astronomers not to overlook the ISM. The ISM can often add clues which will help to solve the puzzles presented by stellar studies. I give as an example HI observations of IZw18. This is a dwarf galaxy thought by some to be experiencing its first burst of star formation. HI observations reveal a plume of neutral gas connecting IZw18 to a companion (Dufour et al. 1996, Skillman et al. 1996a). This suggests that the present burst of star formation has been triggered by a dynamical interaction, and reminds us that sorting out the star formation history of this galaxy must include an understanding of its environment.
Figure 1. A diagram of the photometric and chemical evolution connections between different astronomical entities from Tinsley (1980). This diagram was taken from Knapp (1990) where double-lined boxes emphasize those areas directly relevant to the ISM.
There are many connections between the studies of stars in a galaxy and the studies of its ISM. A quick perusal of Tinsley's famous flow diagram (Figure 1) elucidates some of these for us. When introductory astronomy textbooks discuss the ISM, they usually introduce the topic in relationship to star formation processes. As the ISM is the raw material for star formation, the link is obvious. When I was originally outlining these lectures, I thought that I would dedicate a good fraction of my time to this connection. However, as my thinking evolved, it became clear that this subject is too large to usefully treat as part of a chapter. Indeed, there are now many, many volumes dedicated to this topic. For example, one of the best is the proceedings of the Third Canary Islands Winter School (Tenorio-Tagle, Prieto, & Sanchez 1992), and probably the most recent is the 1996 Maryland Conference proceedings (Holt & Mundy 1997). It also became clear that the emphasis of this school would be on the understanding of stars that already exist in galaxies, and not on their creation.
A second connection is that of chemical evolution. While this is a problem that rivals star formation in its complexity, the connections to existing stars are more clear. The record of the chemical evolution of galaxies is locked into its low mass stars. The present status of chemical evolution is represented in both the youngest stars and the interstellar medium. As a starting point, we might demand that the abundances measured in those two components agree. We will also see the close connections between stellar evolution theory and the interstellar medium as one compares the theoretical nucleosynthetic yields with the "effective" yields one obtains through measurements of gas mass fractions. It is also possible to constrain nuclear cross sections through measurements of relative abundances, again an area where ISM observations can play a role.
Finally, the ISM can act as a recorder, helping us to better understand stellar physics by demonstrating a star's effects on its environment. I can think of two immediate examples concerning massive stars. The first is a subject which Claus covers in detail in his chapter. The energy deposition of massive stars is best measured by the energy content of the surrounding ISM. The second is a topic which will be covered by Rolf. A long standing problem in nebular photoionization modeling (the Ne III problem) has recently been resolved (Sellmaier et al. 1996). The solution to the problem lay in a better understanding of the effects of winds on the radiative transfer in massive stars. Here we have an example of observations of the ISM helping to bring about an advance in stellar atmospheres.
Thus, I have chosen the following goal for this chapter. I hope to provide an entrance point for stellar astronomers into understanding observations of the ISM and how they might be relevant to helping to understand the studies of resolved stars in other galaxies. I will concentrate on the theme of chemical evolution, as I think that this provides the greatest number of links between stellar and ISM studies.
I note here that the problem of understanding the chemical evolution of galaxies is a messy one. There are many important parameters that are coupled in non-linear ways. Thus, the key to success in this field is the isolation of variables. Luckily, we have a wealth of galaxies to study. The variety of galaxies in the Local Group and other relatively nearby groups allows us to test for dependences on various galaxian structural properties. It is important to think of the variety in the nearby galaxies as providing baselines in various parameters against which to test the dependences of observables, and later, theories. I have a clear prejudice for the nearby galaxies; I believe that the resolved stellar populations and interstellar medium in these galaxies will eventually allow us our best insights into the processes that determine galaxy evolution.