The dynamical signature of the ISM in soft X-rays -- I. Diffuse soft X-rays from galaxies

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We present the first dynamically and thermally self-consistent calculations of fast adiabatically expanding gas flows from the Galactic disk into the halo. It is shown that in a hot plasma (T >= 10⁶ K) with a high overpressure with respect to the ambient medium, the dynamical time scale is much shorter than the intrinsic time scales (e.g. for recombination, collisional excitation and ionization etc.). Therefore dynamical models that use collisional ionization equilibrium (CIE) cooling functions for the evolution of the plasma are in general not correct. In particular, the emission spectra obtained from non-equilibrium calculations are radically different. We describe a method to obtain self-consistent solutions using an iterative procedure. It is demonstrated that soft X-ray background emission between 0.3 and 1.5 keV can be well explained by a superposition of line emission and delayed recombination of an initially hot plasma streaming away from the Galactic disk (outflow and/or winds). In addition to these local winds we also present calculations on global winds from spiral galaxies, which originate from a hot and quiescent galactic corona. We also emphasize that it is dangerous to derive plasma temperatures merely from line ratios of ionized species, such as NV/OVI, unless the dynamical and thermal history of the plasma is known.