The mode of gas accretion onto star-forming galaxies
  It is argued that galaxies like ours sustain their star formation by
transferring gas from an extensive corona to the star-forming disc. The
transfer is effected by the galactic fountain -- cool clouds that are shot up
from the plane to kiloparsec heights above the plane. The Kelvin-Helmholtz
instability strips gas from these clouds. If the pressure and the the
metallicity of the corona are high enough, the stripped gas causes a similar
mass of coronal gas to condense in the cloud's wake. Hydrodynamical simulations
of cloud-corona interaction are presented. These confirm the existence of a
critical ablation rate above which the corona is condensed, and imply that for
the likely parameters of the Galactic corona this rate lies near the actual
ablation rate of clouds. In external galaxies trails of HI behind individual
clouds will not be detectable, although the integrated emission from all such
trails should be significant. Parts of the trails of the clouds that make up
the Galaxy's fountain should be observable and may account for features in
targeted 21-cm observations of individual high-velocity clouds and surveys of
Galactic HI emission. Taken in conjunction with the known decline in the
availability of cold infall with increasing cosmic time and halo mass, the
proposed mechanism offers a promising explanation of the division of galaxies
between the blue cloud to the red sequence in the colour-luminosity plane.
