Shrinking Galaxy Disks with Fountain-Driven Accretion from the Halo
  Star formation in most galaxies requires cosmic gas accretion because the gas
consumption time is short compared to the Hubble time. This accretion
presumably comes from a combination of infalling satellite debris, cold flows,
and condensation of hot halo gas at the cool disk interface, perhaps aided by a
galactic fountain. In general, the accretion will have a different specific
angular momentum than the part of the disk that receives it, even if the gas
comes from the nearby halo. Then the gas disk expands or shrinks over time.
Here we show that condensation of halo gas at a rate proportional to the star
formation rate in the fountain model will preserve an initial shape, such as an
exponential, with a shrinking scale length, leaving behind a stellar disk with
a slightly steeper profile of younger stars near the center. This process is
slow for most galaxies, producing imperceptible radial speeds, and it may be
dominated by other torques, but it could be important for Blue Compact Dwarfs,
which tend to have large, irregular gas reservoirs and steep blue profiles in
their inner stellar disks.
