The Need for a Self-Consistent Model of the ISM

Abstract

The interstellar medium of the Milky Way possesses a remarkable range of components, which have individual lifetimes considerably less than a Hubble time. Consequently, there has been a major effort in understanding the properties of these components, their heating and cooling mechanisms, and the process by which one component is transformed into another. We review several of the issues in an effort to highlight controversies that are under study presently. One of the central debates involves star formation, where there are two general and conflicting theoretical models. Another issue deals with the efforts to understand the fractional amounts of gas in the various phases, which brings together star formation with global gas dynamical reactions. This interaction is responsible for heating of the gas by Supernovae, causing superbubbles, and subsequently, either galactic winds or galactic fountains. The relative amounts of material in the hot and cold phases not only depends upon the supernova rate, it also depends upon details of interstellar models, such as whether thermal conduction is an important process. We review the progress that people have made on self-consistent modeling of the ISM. The constraints on such models come from a wide variety of observations of molecular, neutral atomic, warm ionized, and X-ray gas. Both observations and theory are represented at this meeting and we highlight areas in which contributions are likely to resolve the existing controversies.