The motivation for studying X-ray emission from superwinds has always been the hope that the observed thermal X-ray emission provides a direct probe of the hot, metal-enriched, gas that drives these outflows. The 10-kpc-scale diffuse X-ray emission in superwinds seen by Einstein, ROSAT & ASCA had characteristic temperatures of a few million degrees. This is much cooler than the ~ 108 K expected for raw SN-ejecta within the starburst region, but as the wind expands cooling processes such as adiabatic expansion or mass-loading (Suchkov et al. 1996) might reconcile the observed temperatures with an interpretation of the X-ray emission coming from a volume-filling wind of SN-ejecta. The alternative model is that X-ray emission comes from an interaction between the hot, high velocity wind, and cooler denser ISM along the walls of the outflow or in clouds embedded within the wind (Chevalier & Clegg 1985; Suchkov et al. 1994; Strickland & Stevens 2000). A fraction of the cool dense ISM is shock-or-conductively heated to million degree temperatures.
Distinguishing between these two competing models has awaited high spatial resolution X-ray imaging. If the X-ray emission comes from a volume-filling wind, then the X-ray emission should smoothly fill the interior of the cone or lobes outlined by H emission. In the wind/ISM interaction model the X-ray emission should be concentrated in regions with dense cool gas, and it should appear as filamentary and limb-brightened as the H emission. The spatial resolution and sensitivity of X-ray instruments prior to Chandra was not high enough to make an exact comparison between X-ray and H emission in even the nearest superwinds, although a general correlation between the two has long been noted (Watson et al. 1984; McCarthy et al. 1987).
Chandra's 1" spatial resolution corresponds to ~ 12 pc at the distance of nearby superwinds - the same physical scales as the H emitting clouds. We now have unambiguous evidence (Fig. 3) for a 1-to-1 relationship between the spatial distribution of the soft thermal X-ray emission and the H emission in the inner kpc of several superwinds (Strickland et al. 2000; 2001 in preparation). Over the larger 10 kpc scales of the winds the X-ray emission still appears filamentary and arc-like, and is associated with nearby filaments or arcs of H emission. Low-volume filling factor gas dominates the X-ray emission from superwinds, and the the gas that actually drives the outflow remains invisible.
Figure 3. Soft X-ray and H emission in several edge-on starburst galaxies, showing the spatial similarities between the two phases. NGC 253 & NGC 4945 have kpc-scale limb-brightened nuclear outflow cones (the opposite outflow cone is obscured in both cases) with a close match between X-ray & H emission. In NGC 3628 a 5 kpc-long H arc on the eastern limb of the wind is matched by an offset X-ray filament.