Fountains, bubbles, and hot gas

Gas is observed to extend far from the disk of the Galaxy, with a scale height that increases with ionization state of the gas. Recent observations have shown that a substantial amount of the halo gas is hot. This gas could be either heated internally, by halo supernovae for example, and/or it could be energized by gas venting from superbubbles in the Galactic disk—a Galactic fountain. Calculation of the UV emission and absorption spectrum of a Galactic fountain shows good agreement with observation; in the absence of conduction, agreement with x‐ray observations requires fine tuning of the initial temperature of the fountain. The superbubbles that provide the mass and energy of the gaseous halo in fountain models are created by OB associations in the disk. A recent calculation of the luminosity distribution of Galactic OB associations is described. The filling factor of superbubbles in the disk is estimated to be of order 0.1; combined with observations of H I and C IV, this suggests a mass flow of sever...