Self-Consistent Modelling of the Interstellar Medium

Abstract

The dynamical evolution of hot optically thin plasmas in the ISM crucially depends on the heating and cooling processes. It is essential to realize that all physical processes that contribute operate on different time scales. In particular detailed balancing is often violated since the statistically inverse process of e.g. collisional ionization is recombination of an ion with two electrons, which as a three-body collision is usually dominated by radiative recombination, causing a departure from collisional ionization equilibrium. On top of these differences in atomic time scales, hot plasmas are often in a dynamical State, thereby introducing another time scale, which can be the shortest one. The non-equilibrium effects will be illustrated and discussed in the case of galactic outflows. It will be shown, that spectral analyses of X-ray data of edge-on galaxies show a clear signature in the form of ‘multi-temperature’ halos, which can most naturally be explained by the ‘freezing-in’ of highly ionized species in the outflow, which contribute to the overall spectrum by delayed recombination. This naturally leads to a non-equilibrium cooling function, which modifies the dynamics, which in turn changes the plasma densities and thermal energy budget, thus feeding back on the ionization structure. Therefore self-consistent modelling is needed.