Gas circulation and galaxy evolution

Galaxies must form and evolve via the acquisition of gas from the intergalactic environment, however the way this gas accretion takes place is still poorly understood. Star-forming galaxies are surrounded by multiphase halos that appear to be mostly produced by internal processes, e.g., galactic fountains. However, a small fraction of the halo gas shows features that point to an external origin. Estimates of the halo-gas accretion rate in the local Universe consistently give values much lower than what would be required to sustain star formation at the observed rate. Thus, most of the gas accretion must be "hidden" and not seen directly. I discuss possible mechanisms that can cause the intergalactic gas to cool and join the star-forming galactic disks. A possibility is that gas accretion is driven by the galactic-fountain process via turbulent mixing of the fountain gas with the coronal low-metallicity gas.