The Cold Interstellar Medium of Galaxies in the Local Universe

The cold interstellar medium (ISM) plays a central role in the galaxy evolution process. It is the reservoir that fuels galaxy growth via star formation, the repository of material formed by these stars, and a sensitive tracer of internal and external processes that affect entire galaxies. Consequently, significant efforts have gone into systematic surveys of the cold ISM of the galaxies in the local Universe. This review discusses the resulting network of scaling relations connecting the atomic and molecular gas masses of galaxies with their other global properties (stellar masses, morphologies, metallicities, star-formation activity…) and their implications for our understanding of galaxy evolution. Key take-home messages are as follows: ▪ From a gas perspective, there are three main factors that determine the star-formation rate of a galaxy: the total mass of its cold ISM, how much of that gas is molecular, and the rate at which any molecular gas is converted into stars. All three of these factors vary systematically across the local galaxy population. ▪ The shape and scatter of both the star-formation main sequence and the mass–metallicity relation are deeply linked to the availability of atomic and molecular gas. ▪ Future progress will come from expanding our exploration of scaling relations into new parameter space (in particular, the regime of dwarf galaxies), better connecting the cold ISM of large samples of galaxies with the environment that feeds them (the circumgalactic medium, in particular), and understanding the impact of these large scales on the efficiency of the star-formation process on molecular cloud scales