The Evolving ISM in the Milky Way & Nearby Galaxies ISM Theory: Gas Phases, Turbulence, and Star Formation

The ISM is a complex and highly dynamic system, and recent numerical modeling efforts have begun to take into account the effects of multiscale, timedependent processes, generically classified as “turbulence”. Turbulence transforms what would otherwise be a simple two-temperature state for atomic gas into a broader, but still bimodal, distribution; it also affects the relative proportions of gas in each phase – including the molecular phase – by collecting gas in large-scale shocks and by increasing the gas scale height, which determines the midplane pressure. Turbulence has many sources, both from stars and from tapping the rotational energy in galaxies (through magnetic effects and other instabilities). Star formation is strongly affected by turbulence as well, in both “positive” (increased compression in shocks) and “negative” (decreased mean densities from turbulent pressure) ways. In addition to turbulence, “environmental” factors including spiral structure strongly affect the state of the ISM, and the regulation of star formation. I discuss recent advances in modeling, as well as open questions for future theoretical and observational investigations. Subject headings: galaxies: ISM — ISM: kinematics and dynamics — turbulence — stars: formation — ISM: structure