Modelling the supernova-driven ISM in different environments

Abstract

We use hydrodynamical simulations in a $(256\mathrm{pc}{)}^3$ periodic box to model the impact of supernova (SN) explosions on the multi-phase 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 ($T\gtrsim 10^6$ K) filling $>90$% of the volume. This gas reaches high pressures ($10^4<P/k_{\mathrm{B}}<10^7$ 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 ($>50$%), 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 ($\ll 1$%). 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 $\lesssim 10$ km s${}^{-1}$ in all cases. For peak driving the velocity dispersion in H${}_{\hat{I}}\pm$ can be as high as $70$ km s${}^{-1}$ due to the contribution from young, embedded SN remnants.

19 pages, 12 figures, 2 tables. Accepted for publication in MNRAS. Minor revisions to match published version