Whats the reason for the galactic fountain?
When a bunch of supernovae happen in a star cluster at about the same time, a gigantic hot bubble of gas (a superbubble) is formed. When this happens in the Galactic disk, the superbubble’s expansion is slowed due to the presence of interstellar gas already in the disk. But once the superbubble’s expansion reaches the edge of the disk, and the superbubble encounters less gas, it can “blow out” of the disk and into the halo. A stream of hot gas, like a geyser or volcano, spews out- this is called a “fountain”. Eventually, the gas will expand and cool, and due to gravity will fall back down, “raining” back onto the disk.