Time-dependent cooling in astrophysical plasmas: The Non-equilibrium Ionization Structure of the Interstellar Medium and X-ray Emission at Low Temperatures

Abstract

The interstellar medium (ISM) is a dynamical system, in which the plasma is naturally driven out of ionization equilibrium due to atomic and dynamic processes operating on different timescales. We report on the first to date 0.5 pc resolution three-dimensional hydrodynamical simulation of the ISM, including the disk–halo interaction, and featuring a detailed time-dependent calculation of the non-equilibrium ionization structure. In particular, we study the effects of the history of the plasma on the cooling functions and determine the associated X-ray emission at low temperatures. The main results of this work are: (1) in a dynamical ISM, the time-dependent ionization structure, and therefore the cooling function, varies in space and time depending on the initial conditions and its history; (2) the cooling paths can be quite different for gas with the same initial temperature, but having different evolution histories; and (3) due to delayed recombination in a dynamic plasma, X-ray emission can occur at low temperatures becoming, eventually, stronger than the corresponding emission from a plasma in collisional ionization equilibrium at 106.2 K. This has far-reaching consequences for the interpretation of EUV/X-ray spectra. We emphasize that these results can be generalized and applied to any astrophysical system, in which radiative cooling is relevant.