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 |