  Galactic outflows of low ionization, cool gas are ubiquitous in local
starburst galaxies, and in the majority of galaxies at high redshift. How these
cool outflows arise is still in question. Hot gas from supernovae has long been
suspected as the primary driver, but this mechanism suffers from its tendency
to destroy the cool gas as the latter is accelerated. We propose a modification
of the supernova scenario that overcomes this difficulty.
  Star formation is observed to take place in clusters; in a given galaxy, the
bulk of the star formation is found in the ~20 most massive clusters. We show
that, for L* galaxies, the radiation pressure from clusters with M>10^6 M_sun
is able to expel the surrounding gas at velocities in excess of the circular
velocity of the disk galaxy. This cool gas can travel above the galactic disk
in less than 2 Myr, well before any supernovae erupt in the driving cluster.
Once above the disk, the cool outflowing gas is exposed to radiation, and
supernovae induced hot gas outflows, from other clusters in the disk, which
drive it to distances of several tens to hundreds of kpc. Because the
radiatively driven clouds grow in size as they travel, and because the hot gas
is more dilute at large distance, the clouds are less subject to destruction if
they do eventually encounter hot gas. Therefore, unlike wind driven clouds,
radiatively driven clouds can survive to distances ~50 kpc. We identify these
cluster-driven winds with large-scale galactic outflows. Another implication of
our model is that only starburst galaxies, where massive clusters reside, are
able to drive winds cold outflows on galactic scales via this mechanism. We
find that the critical star formation rates above which large scale cool
outflows will be launched to be ~0.1 M_sun/yr/kpc^2, which is in good agreement
with observations.
