Introduction

The ISM contains a number of phases characterized by different temperatures, densities, and ionization fractions (cf. Section 1.2 and Table 1.1). The origin and interrelationship of these phases in the ISM, and their energy and ionization sources, are among the most fundamental subjects of investigation in the field. Quite generally, a new stable phase reflects the onset of a new cooling mechanism or the decline of a heating source. Hence, cold HI clouds and the warm intercloud medium result from the increased importance of [CII] cooling at higher densities and Lyα and [OI] 6300Å cooling at higher temperatures, respectively (cf. Section 2.6.1). The hot phase reflects the recent input of supernova energy (cf. Section 12.3). Cold molecular clouds result from the increased cooling due to rotational transitions in molecules (cf. Section 2.6.2). However, the latter fall somewhat outside this classification scheme since their existence also reflects the importance of self-gravity. Understanding the structure of the ISM thus requires an understanding of its sources of heating and cooling.

Heating and cooling processes have been discussed in Chapters 3 and 2, respectively. Here, we apply these to the neutral diffuse interstellar medium. We will first examine the ionization and energy balance of the cold and warm phases of the interstellar medium (Section 8.2). Then, we will focus on the physical principles that allow several phases to coexist in thermal and pressure equilibrium in the interstellar medium (Section 8.3).