Cold Exponential Disks From Interstellar Fountains
  We present the results of a simple numerical model with phenomenological
cloud growth and explosive disruption processes, and with fountain launched
ballistic motions of disrupted cloud fragments out of the disk. These processes
generate an effective scattering of gas elements over much larger distances
than noncircular impulses in the plane, which are quickly damped. The result is
evolution of the global cloud density profile to an exponential form on a
roughly Gyr timescale. This is consistent with our previous results on the
effects of star scattering o? massive clumps in young disks, and gas holes in
dwarf galaxies. However, in those cases the scattering processes generated
thick, warm/hot stellar disks. Here we find that the exponential gas disks
remain cold. Star formation in this gas would produce a thin exponential
stellar disk.
