Published in "The Environment and Evolution of Galaxies", eds. J.M. Shull and A. Thronson, Jr., 1993
Abstract. We review the galactic 'superwind' phenomenon: global outflows powered by the collective effect of the supernova explosions and stellar winds associated with powerful, compact starbursts. We briefly summarize the theoretical and astrophysical underpinnings of the superwind concept, stressing how even simple idealized models can provide an informative interpretational guide to the observations.
We then discuss the growing body of multi-waveband observations of superwinds. X-ray emission from superwinds is probably produced by a combination of internal shocks in the high-speed superwind and by interactions between the superwind and inhomogeneous ambient gas ('clouds'). Most of the available data on superwinds comes from optical emission-line imaging and spectroscopy. Such optical line-emission is most likely produced as the wind shock-heats and accelerates ambient gas in the disk or halo of the starburst galaxy. These optical data provide morphological, kinematic, and physical evidence for the existence of superwinds, and even allow us to estimate some of the key superwind parameters. The interaction between the wind and cool atomic gas has been probed by both optical absorption-lines and radio observations of the H I 21cm line. Evidence for outflowing molecular gas in starbursts is also summarized. The nonthermal radio continuum halos associated with some starbursts probably represent relativistic magnetized plasma convected out of the starburst by the superwind.
We then summarize our attempts to take a census of superwinds in the local universe. We use this census to estimate the rates at which superwinds eject metals and energy at the present epoch. Finally, we briefly consider the implications of superwinds for diverse topics in extragalactic astronomy. Superwinds may play a crucial role in the chemothermal evolution of the intergalactic medium, in the chemical evolution of galaxies, in the evolution (destruction?) of dwarf galaxies, in the stimulation or suppression of galaxy formation at early times, in the quasar absorption-line phenomenon (through their effect on galactic gaseous halos), and in the production of part of the X-ray background at E < 10 keV.
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