Radiation Pressure Driven Galactic Winds from Self-Gravitating Discs
  (Abridged) We study large-scale winds driven from uniformly bright
self-gravitating discs radiating near the Eddington limit. We show that the
ratio of the radiation pressure force to the gravitational force increases with
height above the disc surface to a maximum of twice the value of the ratio at
the disc surface. Thus, uniformly bright self-gravitating discs radiating at
the Eddington limit are fundamentally unstable to driving large-scale winds.
These results contrast with the spherically symmetric case, where
super-Eddington luminosities are required for wind formation. We apply this
theory to galactic winds from rapidly star-forming galaxies that approach the
Eddington limit for dust. For hydrodynamically coupled gas and dust, we find
that the asymptotic velocity of the wind is v_\infty ~ 1.5 v_rot and that
v_\infty SFR^{0.36}, where v_rot is the disc rotation velocity and SFR is the
star formation rate, both of which are in agreement with observations. However,
these results of the model neglect the gravitational potential of the
surrounding dark matter halo and an old passive stellar bulge or extended disc,
which act to decrease v_\infty. A more realistic treatment shows that the flow
can either be unbound, or bound, forming a "fountain flow" with a typical
turning timescale of t_turn ~ 0.1-1 Gyr. We provide quantitative criteria and
scaling relations for assessing whether or not a rapidly star-forming galaxy of
given properties can drive unbound flows via the mechanism described in this
paper. Importantly, we note that because t_turn is longer than the star
formation timescale in the rapidly star-forming galaxies and ULIRGs for which
our theory is most applicable, if rapidly star-forming galaxies are selected as
such, they may be observed to have strong outflows, even though their winds are
eventually bound on large scales.
