MR. F.A. GENT

The multi-phase interstellar medium (ISM) randomly heated and stirred by supernovae (SN), with gravity, differential rotation and other parameters of the solar neighbourhood have been simulated. With a three-dimensional domain extending 1 x 1 kpc^2 horizontally and 2 kpc vertically (symmetric about the galactic mid-plane), the model routinely spans gas number densities 10^-5–10^2 cm^-3, temperatures 10–10^8 K, local velocities up to 103 kms^-1 (with Mach number up to 25). Analysis of the thermal structure of the modelled ISM confirms that most of the complexity can be captured in terms of just three phases, separated by temperature borderlines of about 10^3 K and 5 x 10^5 K. The distribution of gas densities within each phase is approximately lognormal. The connection between the fractional volume of a phase and its various proxies is clarified, and an exact relation between the fractional volume and the filling factors defined in terms of the volume and probabilistic averages has been derived. The growth and structure of the magnetic field in spiral galaxies is investigated by including non-ideal MHD. The magnetic field is evolved from a seed field for Galaxies with parameters modelled on the solar neighbourhood and similar, each indicating the presence of a dynamo. Exponential growth of the magnetic energy occurs with turnover times exceeding 1.0 Gyr to saturate at levels consistent with the mean kinetic energy density in the models and at a field strength consistent with estimates in the solar neighbourhood. The resultant field has a significant large scale structure and strong fluctuations and the nature of the mean and fluctuation dynamo is discussed. The decomposition of the total field into mean and random parts is considered in the kinematic and saturated states of the simulations.