OVI Emission From the Supernovae-regulated Interstellar Medium: Simulation Vs Observation

Abstract

The OVI $\lambda\lambda$1032, 1038\AA\ doublet emission traces collisionally ionized gas with $T\approx 10^{5.5}$ K, where the cooling curve peaks for metal-enriched plasma. This warm-hot phase is usually not well-resolved in numerical simulations of the multiphase interstellar medium (ISM), but can be responsible for a significant fraction of the emitted energy. Comparing simulated OVI emission to observations is therefore a valuable test of whether simulations predict reasonable cooling rates from this phase. We calculate OVI $\lambda$1032\AA\ emission, assuming collisional ionization equilibrium, for our small-box simulations of the stratified ISM regulated by supernovae. We find that the agreement is very good for our solar neighborhood model, both in terms of emission flux and mean OVI density seen in absorption. We explore runs with higher surface densities and find that, in our simulations, the OVI emission from the disk scales roughly linearly with the star formation rate. Observations of OVI emission are rare for external galaxies, but our results do not show obvious inconsistency with the existing data. Assuming the solar metallicity, OVI emission from the galaxy disk in our simulations accounts for roughly 0.5\% of supernovae heating.