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Observations of edge-on starburst galaxies show weakly collimated 10 kpc-outflows of gas (Fig. 1), with outflow velocities of several hundred kilometers per second (McCarthy, Heckman, & van Breugel 1987; Heckman, Armus, & Miley 1990; Armus, Heckman, & Miley 1990). Tracers of warm ionized gas such as Halpha emission show filaments and arcs of emission extending outward from the nuclear regions of the host galaxy galaxy, which outline the surfaces of bipolar outflow cones of opening angle theta ~ 60deg. The primary observational probes of these outflows have historically been been optical emission lines (Armus et al. 1990), and X-ray emission (Dahlem, Weaver, & Heckman 1998), although all phases of the ISM have been detected (Dahlem 1997). The X-ray emission correlates well spatially with the Halpha emission (see Fig. 1), although in many cases the X-ray observations trace these outflows out to larger galactocentric radii (ltapprox 20 kpc, Read, Ponman, & Strickland 1997) than the Halpha observations.

Figure 1a Figure 1b

Figure 1. (a) Narrowband Halpha+continuum image of the nearby (D = 3.63 Mpc) edge-on starburst galaxy M82, which shows filaments of 104 K gas flowing out of the galaxy at ~ 600 km/s along the minor axis. The white crosses mark the positions of ~ 50 young SNRs detected in radio observations of the central 800 × 100 pc starburst region. (b) A soft X-ray Chandra ACIS-I image of M82, showing gas with characteristic temperature of a few million degrees.

These outflows result from the energy returned to the ISM by the recently-formed massive stars in the starburst. Core collapse supernovae and massive star stellar winds, from ~ 106 O & B stars in galaxies like M82 or NGC 253, return large amounts of kinetic energy along with metal-enriched ejecta to the ISM. Radio observations of local starbursts reveal large numbers of young SNRs within the starburst region (Kronberg, Biermann, & Schwab 1981; Muxlow et al. 1994). Age estimates for the starburst stellar populations (~ 10 Myr for M82, 20 - 30 Myr in NGC 253 [Satyapal et al. 1997; Engelbracht et al. 1998]) agree well with the dynamical ages of the outflows, taudyn ~ 10 kpc/500 km/s ~ 20 Myr. The kinetic energy of the individual remnants and wind-blown bubbles is thermalized via shocks as SNRs overlap and interact, creating a hot (T ~ 108 K), high pressure (P/k ~ 107 K cm-3) bubble of metal-enriched gas in the starburst region (Chevalier & Clegg 1985). This "superbubble" expands preferentially along the path of least resistance (i.e. lowest density), breaking out of the disk of the galaxy along the minor axis after a few million years. The hot gas then expands at higher velocity (v gtapprox 1000 km/s) into the low density halo of the galaxy as a superwind, dragging along clumps and clouds of cool dense entrained ISM at lower velocity (see Suchkov et al. 1994).

Many excellent reviews of both observations and theory of starburst-driven superwinds already exist (Heckman, Lehnert, & Armus 1993; Heckman 1998). In this contribution I highlight recent results related to the issue of mass, metal and energy transport by superwinds out of galaxies and into the IGM.

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