The Need for a Self-Consistent Model of the ISM

Abstract

The interstellar medium of the Milky Way possesses a remarkable rangeof components, which have individual lifetimes considerably less than aHubble time. Consequently, there has been a major effort inunderstanding the properties of these components, their heating andcooling mechanisms, and the process by which one component istransformed into another. We review several of the issues in an effort tohighlight controversies that are under study presently. One of the centraldebates involves star formation, where there are two general andconflicting theoretical models. Another issue deals with the efforts tounderstand the fractional amounts of gas in the various phases, whichbrings together star formation with global gas dynamical reactions. Thisinteraction is responsible for heating of the gas by supernovae, causingsuperbubbles, and subsequently, either galactic winds or galacticfountains. The relative amounts of material in the hot and cold phasesnot only depends upon the supernova rate, it also depends upon details ofinterstellar models, such as whether thermal conduction is an importantprocess. We review the progress that people have made onself-consistent modeling of the ISM. The constraints on such models comefrom a wide variety of observations of molecular, neutral atomic, warmionized, and X-ray gas. Both observations and theory are represented atthis meeting and we highlight areas in which contributions are likely toresolve the existing controversies.