A Dynamic Interstellar Medium: Recent Numerical Simulations

Recent numerical simulations of the interstellar medium driven by energy input from supernovae and stellar winds indicate that H I clouds can be formed by compression in shock waves and colliding turbulent streams without any help from thermal and gravitational instabilities. The filling factor of the hot phase in these models does not exceed 20–40% at the midplane. The hot gas is involved in a systematic vertical outflow at |z| < 1–3 kpc, similar to that expected for galactic fountains, whereas the warm component may remain in hydrostatic equilibrium. The turbulent velocity is larger in the warmer phases, being 3, 10 and 40 km s in the cool, warm and hot phases, respectively, according to one of the simulations. The models exhibit global variability in the total kinetic and thermal energy and star formation rate at periods of (0.4–4) × 10 yr. Current models are still unable to reproduce dynamo action in the interstellar gas; we briefly discuss implications of the dynamo theory for turbulent interstellar magnetic fields.