The Hot Gas Filling Factor in Our Galaxy

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Abstract
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 field. Globally, the hot gas is found to occupy a rather small fraction of the interstellar volume. At the solar circle, this fraction is 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 ~ 6.7 kpc, |Z| ~ 260 pc) entirely filled with hot gas.