1. INTRODUCTION

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 H 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 ~ 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 H emission (see Fig. 1), although in many cases the X-ray observations trace these outflows out to larger galactocentric radii ( 20 kpc, Read, Ponman, & Strickland 1997) than the H observations.

Figure 1. (a) Narrowband H+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, _dyn ~ 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 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.