The Interstellar Medium

The interstellar medium (ISM) is the material that fills the space between the stars in all galaxies; it is a multi-phase medium in pressure equilibrium, with densities and temperatures covering over 6 orders of magnitude. Although accounting for only a small fraction of the mass of any given galaxy, it is a vital component, since it holds the material responsible for galaxy growth through star formation. Studying the ISM requires careful observations at all wavelengths of the electromagnetic spectrum. This article describes the multi-phase nature of the ISM, and then puts it in the context of galaxy evolution models, emphasising the importance of the cycling of baryons in and out of galaxies. Within this framework, the ISM plays a central role: it connects the physical processes operating on very large physical- and time-scales which control the accretion of gas onto galaxies, and the small scale processes that regulate star formation.