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.
