The roles of radiation and ram pressure in driving galactic winds
  We study gaseous outflows from disk galaxies driven by the combined effects
of ram pressure on cold gas clouds and radiation pressure on dust grains.
Taking into account the gravity due to disk, bulge and dark matter halo, and
assuming continuous star formation in the disk, we show that radiation or ram
pressure alone is not sufficient to drive escaping winds from disk galaxies,
and that both processes contribute. We show that in the parameter space of star
formation rate (SFR) and rotation speed of galaxies, the wind speed in galaxies
with rotation speed $v_c\le 200$ km s$^{-1}$ and SFR $\le 100$ M$_{\odot}$
yr$^{-1}$, has a larger contribution from ram pressure, and that in high mass
galaxies with large SFR, radiation from the disk has a greater role in driving
galactic winds. The ratio of wind speed to circular speed can be approximated
as ${v_w \over v_c} \sim 10^{0.7} \, [{\rm SFR\over 50 \, {\rm M}_{\odot} \,
{\rm yr}^{-1}}] ^{0.4} \ [{v_c\over 120\, km/s}]^ {-1.25}$. We show that this
conclusion is borne out by observations of galactic winds at low and high
redshift and also of circumgalactic gas. We also estimate the mass loading
factors under the combined effect of ram and radiation pressure, and show that
the ratio of mass loss rate to SFR scales roughly as $v_c^{-1} \Sigma_g^{-1}$,
where $\Sigma_g$ is the gas column density in the disk.
