Modelling the supernova-driven ISM in different environments

We use hydrodynamical simulations in a (256 pc)(3) periodic box to model the impact of supernova (SN) explosions on the multiphase interstellar medium (ISM) for initial densities n = 0.5-30 cm(-3) and SN rates 1-720 Myr(-1). We include radiative cooling, diffuse heating, and the formation of molecular gas using a chemical network. The SNe explode either at random positions, at density peaks, or both. We further present a model combining thermal energy for resolved and momentum input for unresolved SNe. Random driving at high SN rates results in hot gas (T greater than or similar to 10(6) K) filling > 90 per cent of the volume. This gas reaches high pressures (10(4) < P/k(B) < 10(7) K cm(-3)) due to the combination of SN explosions in the hot, low-density medium and confinement in the periodic box. These pressures move the gas from a two-phase equilibrium to the single-phase, cold branch of the cooling curve. The molecular hydrogen dominates the mass (> 50 per cent), residing in small, dense clumps. Such a model might resemble the dense ISM in high-redshift galaxies. Peak driving results in huge radiative losses, producing a filamentary ISM with virtually no hot gas, and a small molecular hydrogen mass fraction (<< 1 per cent). Varying the ratio of peak to random SNe yields ISM properties in between the two extremes, with a sharp transition for equal contributions. The velocity dispersion in HI remains less than or similar to 10 km s(-1) in all cases. For peak driving, the velocity dispersion in H alpha can be as high as 70 km s(-1) due to the contribution from young, embedded SN remnants.