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We have reviewed the theoretical basis for, the observational evidence pertaining to, and the astrophysical implications of starburst-driven galactic superwinds: global outflows driven by the collective effect of the mechanical energy supplied by supernovae and stellar winds in a nuclear or circumnuclear starburst.

Models of the superwind phenomenon imply that a starburst can create a highly over-pressured bubble of hot gas which will expand preferentially along the direction of the maximum pressure gradient in the ambient ISM of the starburst galaxy (i.e., probably along the galaxy minor axis). For typical starburst conditions, it is expected that this expanding bubble will quickly 'blow-out' into the galactic halo or beyond to form a high-speed (several thousand km s-1) wind.

Hard X-rays (kT = 5 - 10 keV) will be produced by the hot fluid inside the wind's sonic radius and by internal shocks at larger radii. Much softer X-rays can arise as the wind shock-heats and/or evaporates dense ambient gas in the galactic halo. The available X-ray imaging and spectroscopic data on starburst galaxies are broadly consistent with this simple picture, though the weakness of the 6.7 keV Fe K line is a puzzle.

Optical line emission will also be produced as dense halo gas is shock-heated and/or evaporated by the wind. Optical data on starbursts spanning nearly three orders-of-magnitude in luminosity provide highly suggestive evidence for the ubiquity of superwinds. There is a marked excess of emission-line gas along the minor axes of starburst galaxies viewed edge-on. The gas along the minor axis is much more disturbed kinematically than that along the major axis, the line widths along the minor axis correlate better with the starburst luminosity than with the galaxy rotation speed, and the largest velocity shears along the minor axis are observed in the galaxies that are more face-on and have larger IR luminosities (all suggesting a starburst-driven outflow rather than simple orbital motions). In the best-studied (most-nearby) starbursts, the kinematics suggest an outflow along the walls of a cone-like structure at velocities of a few hundred km s-1. The emission-line ratios produced by this gas are consistent with shock-heating, and the large measured pressures agree well with the predicted ram pressure of a superwind. In one case, the outflowing gas has been detected in optical absorption-lines (providing unambiguous evidence for an outflow).

Radio continuum data on a few starburst galaxies imply that relativistic particles and magnetic fields are probably being convected out in the wind at velocities of a few thousand km s-1. Molecular-line observations of superwinds have been reported, and the HI morphology of a companion to the well-known starburst/superwind galaxy NGC 3079 is strongly suggestive of the ablation of the ISM in this galaxy by the ram pressure of the NGC 3079 superwind. HI observations of starburst galaxies show that many such systems are surrounded by large 'envelopes' of H I, presumably of tidal origin. The interaction between the superwind and such relatively dense and cool material probably plays a fundamental role in producing the observed optical line emission and X-rays.

The results from our on-going survey of JR-selected edge-on disk galaxies imply that superwinds are probably ubiquitous in galaxies with warm far-IR colors and total IR luminosities greater than about 1044 erg s-1. The local IR luminosity function of galaxies then allows us to estimate the local injection rate of metals and energy via superwinds. Integrated over a Hubble time (with no cosmic evolution) and normalized to the present space density of all galaxies, we find that superwinds may have ejected a mass of metals comparable to the mass of metals contained inside galaxies and supplied an average of about 1059 ergs per galaxy to intergalactic medium. These figures could easily be an order-of-magnitude larger allowing for evolution of the superwind rate with cosmic time.

Superwinds may play a key role in heating and chemically enriching the intra-cluster and intergalactic media (especially during the epoch of galaxy formation). The 'feedback' between star-formation and the gaseous component of proto-galaxies (via the superwind phenomenon) may have helped imprint the basic and specific properties of galaxies we see today. Superwinds may also have stimulated or suppressed galaxy formation in the environs of newly-formed galaxies. Superwinds may be implicated in the QSO absorption-line phenomenon, as they offer a natural way to create a cosmically evolving population of large, metal-enriched, kinematically-complex gaseous halos. Superwinds also represent an ideal laboratory to study the 'disk-halo connection' under relatively extreme conditions. Superwinds probably contribute non-negligibly to the X-ray background at E < 10 keV. Finally, the superwind phenomenon blurs the empirical distinction between AGNs and starbursts: a starburst with a strong superwind can exhibit broad emission-lines, LINER emission-line ratios, (weak) X-ray emission, and even (weak) large-scale, (weakly) bipolar nonthermal radio emission (all of which are commonly taken to indicate the presence of an AGN).

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