Galactic fountains and extended H I disks

Some of the properties of outer galactic fountains, hot dynamic flows which reach the outer disks of spiral galaxies after some supernova blast waves break through the inner stellar disk with more than average specific angular momentum, are outlined. Numerical simulations of steady downward flows onto the galactic plane are performed for a wide range of mass flow rates and injection temperatures. Three different values for the extragalactic flux of soft X-rays are considered. The buildup of an H I disk is discussed, showing that irregularities in the flow are oscillatory rather than secularly unstable. Conjectures on future models with heat conduction and cases without a steady cooling flow are considered. The strong effect which even a weak galactic fountain can have on the shape of the outer edge of an H I disk is examined. Evaporative flows in young galaxies, which shrink the overall H I disk and may be relevant for some quasar absorption lines, are addressed. 49 references