The hot gas filling factor in our galaxy

Proceeding from the assumption that the hot interstellar gas component in our Galaxy is produced by supernova explosions and the associated stellar winds, we calculate its filling factor as a function of Galactic radius and height. The calculation is carried out numerically by following the time evolution of a realistic distribution of isolated supernova remnants and superbubbles in a stratified medium that includes thermal gas, cosmic rays, magnetic fields, and a gravitational held. Globally, the hot gas is found to occupy a rather small fraction of the interstellar volume. At the solar circle, this fraction is less than or similar to 20% at low altitude and falls off gradually above the dense gas layer. The outer Galaxy remains largely devoid of hot gas, whereas the inner Galaxy may contain a large-scale region (around R similar to 6.7 kpc, \Z\ similar to 260 pc) entirely filled with hot gas.