SIMULATIONS OF THE SUPERNOVA-DOMINATED INTERSTELLAR-MEDIUM IN DISK GALAXIES

preventing the establishment of long-lived "tunnel" networks of hot rarefied gas. remnants (SNRs). We have developed a computer stimulation that uses standard SNR evolutionary solutions to redistribute mass and energy throughout a rectangular, three-level grid. This model includes bremsstrahlung or metal cooling, creation and evaporation of clouds, mass injection to and return from a galactic halo, multiple correlated SNRs, and internally determined SNR lifetimes; we neglected magnetic fields and cooling by dust. We have confirmed the suspicion that spatial correlation of supernova sites can increase the global evolution rate of the metal-cooled ISM. Porosity theory is unable to predict the topology of the interstellar medium because it does not account for mass transport, averaging 0.007 M. pc-2 Myr-1 and 2 M. pc-2 into the halo. In "primeval" (zero metal abundance) galaxies, the enhanced mass transport by bremsstrahlung-cooled remnants continually agitates the ISM, preventing the establishment of long-lived "tunnel" networks of hot rarefied gas. One cannot use the simple SNR number-radius relation to infer evolutionary histories, because of its implicit assumptions of a homogeneous mass density and constant SNR lifetimes; a simple N(R) histogram conveys more information. If SNR shells are the main source of cold clouds in present-day galaxies, the initial size of the clouds can determine the global structure of the ISM. With small clouds (<< 1 pc) that are easily evaporated, less than 25% of interstellar material will be "locked up" in atomic hydrogen, leaving most of the material in a warm or hot phase at high pressure. If the created clouds are of the more usually assumed size (approximately 1 pc), about 70% of the disk mass will remain in cold, cloudy state, and the pressure assumes more conventional interstellar values.