Ionized gas in the halos of edge-on infrared-bright galaxies: Evidence for starburst-driven superwinds

Large-scale, starburst-driven outflows from galaxies ("superwinds") have a rich legacy in theoretical astrophysics and have been invoked to explain many aspects of galactic formation and evolution. Such winds should arise when the supernova rate is high enough to create a cavity of very hot shock-heated gas within a galaxy (Chevalier and Clegg 1985). This gas can then expand outward as a high speed wind that can accelerate and heat ambient interstellar or circum-galactic gas causing it to emit optical line radiation and/or thermal X-rays. Theory suggests that such winds should be common in starburst galaxies and the nature of the winds should depend on the star-formation rate and distribution.; In order to systematize our observational understanding of superwinds (determine their incidence rate and the dependence of their properties on the star-formation that drives them) and to make quantitative comparisons with the theory of superwinds, we have analyzed data from an optical spectroscopic and narrow-band imaging survey of an infrared flux-limited sample of about 50 IR-warm, starburst galaxies whose stellar disks are viewed nearly edge-on. This sample contains galaxies with infrared luminosities from {dollar}{lcub}approx{rcub}10sp{lcub}10-12{rcub} Lsbodot{dollar} and allows us to determine the properties of superwinds over a wide range of star-formation rates. We have found that extra-planar emission-line gas is a very common feature of these edge-on, IR-bright galaxies and the properties of the extended emission-line gas are quantitatively consistent with the superwind theory. These properties are: radial pressure profiles that fall as approximately r{dollar}sp{lcub}-2{rcub},{dollar} pressures in the nuclei of these galaxies that are 3 orders of magnitude higher than the ambient pressure in the ISM of our galaxy, line ratios becoming more shock-like as one proceeds out along the galaxy minor axis, extra-planar filamentary and shell-like emission-line morphologies on scales of 100s of pc to 10s of Kpc, and the kinematic signatures of an outflow whose velocity is related to the star-formation rate.