Much of the interstellar medium in disk galaxies is in the form of neutral atomic hydrogen, H I. This gas can be in thermal equilibrium at relatively low temperatures, T < ∼ 300 K (the cold neutral medium, or CNM) or at temperatures somewhat less than 10 4 K (the warm neutral medium, or WNM). These two phases can coexist over a narrow range of pressures, P min ≤ P ≤ P max . We determine P min and P max in the plane of the Galaxy as a function of Galactocentric radius R using recent determinations of the gas heating rate and the gas phase abundances of interstellar gas. We provide an analytic approximation for P min as a function of metallicity, far-ultraviolet radiation field, and the ionization rate

This work provides a comprehensive overview of our theoretical and observational understanding of the interstellar medium of galaxies. With emphasis on the microscopic physical and chemical processes in space, and their influence on the macroscopic structure of the interstellar medium of galaxies, the book includes developments in this area of molecular astrophysics. The various heating, cooling, and chemical processes relevant for the rarefied gas and submicron-sized dust grains that constitute the interstellar medium are discussed in detail. This provides a firm foundation for an in-depth understanding of the ionized, neutral atomic, and molecular phases of the interstellar medium. The physical and chemical properties of large polycyclic aromatic hydrocarbon molecules and their role in the interstellar medium are highlighted, and the physics and chemistry of warm and dense photodissociation regions are discussed. This is an invaluable reference source for advanced undergraduate and graduate students, and research scientists.

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Large polycyclic aromatic hydrocarbon (PAH) molecules carry the infrared (IR) emission features that dominate the spectra of most galactic and extragalactic sources. This review surveys the observed mid-IR characteristics of these emission features and summarizes laboratory and theoretical studies of the spectral characteristics of PAHs and the derived intrinsic properties of emitting interstellar PAHs. Dedicated experimental studies have provided critical input for detailed astronomical models that probe the origin and evolution of interstellar PAHs and their role in the universe. The physics and chemistry of PAHs are discussed, emphasizing the contribution of these species to the photoelectric heating and the ionization balance of the interstellar gas and to the formation of small hydrocarbon radicals and carbon chains. Together, these studies demonstrate that PAHs are abundant, ubiquitous, and a dominant force in the interstellar medium of galaxies.