Neutral hydrogen in galactic fountains

Simulations of an isolated Milky Way-like galaxy, in which supernovae power a galactic fountain, reproduce the observed velocity and 21-cm brightness statistics of galactic neutral hydrogen (HI). The simulated galaxy consists of a thin HI disc, similar in extent and brightness to that observed in the Milky Way, and extra-planar neutral gas at a range of velocities due to the galactic fountain. Mock observations of the neutral gas resemble the HI flux measurements from the Leiden-Argentine-Bonn HIsurvey, including a high-velocity tail which matches well with observations of high-velocity clouds. The simulated high-velocity clouds are typically found close to the galactic disc, with a typical line-of-sight distance of 13 kpc from observers on the solar circle. The fountain efficiently cycles matter from the centre of the galaxy to its outskirts at a rate of around 0.5M_solaryr^-1.