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Theory and Models of the Disc-Halo Connection

We review the evolution of the interstellar medium in disc galaxies, and show, both analytically and by numerical 3D hydrodynamic simulations, that the disc-halo connection is an essential ingredient in understanding the evolution of star forming galaxies. Depending on the star formation rate of the underlying gaseous disc, a galactic fountain is established. If the star formation rate is sufficiently high and/or cosmic rays are well coupled to the thermal plasma, a galactic wind will be formed and lead to a secular mass loss of the galaxy. Such a wind leaves a unique imprint on the soft X-ray spectra in edge-on galaxies, with delayed recombination being one of its distinctive features. We argue that synthetic spectra, obtained from self-consistent dynamical and thermal modelling of a galactic outflow, should be treated on an equal footing as observed spectra. We show that it is thus possible to successfully fit the spectrum of the starburst galaxy NGC 3079.