Volume Filling Factors of the ISM Phases in Star Forming Galaxies - I. The Role of the Disk-Halo Interaction

The role of matter circulation between the disk and halo in establishing the volume filling factors of the different ISM phases in the Galactic disk is investigated, using a modified version of the three-dimensional supernova-driven ISM model of Avillez (2000). We carried out adaptive mesh refinement simulations of the ISM with five supernova rates (in units of the Galactic value), $\sigma/\sigma_{Gal}=1$, 2, 4, 8 and 16 (corresponding to starburst conditions) using three finer level resolutions of 2.5, 1.25 and 0.625 pc, allowing us to understand how resolution would affect the volumes of gas phases in pressure equilibrium. We find that the volume filling factors of the different ISM phases depend sensitively on the existence of a duty cycle between the disk and halo acting as a pressure release valve for the hot (T> 10^5.5 K) phase in the disk.