Chandra observations of NGC 253:: New insights into the nature of starburst-driven superwinds

Arcsecond-resolution X-ray imaging of the nucleus of the nearby starburst galaxy NGC 253 with Chandra reveals a well-collimated, strongly limb-brightened, kiloparsec-scale conical outflow from the central starburst region. The outflow is very similar in morphology to the known H alpha outflow cone, on scales down to less than or similar to 20 pc. This provides, for the first time, robust evidence that both X-ray and H alpha emission come from low volume filling factor regions of interaction between the fast energetic wind of SN ejecta and the denser ambient interstellar medium and not from the wind fluid itself. We provide estimates of the (observationally and theoretically important) filling factor of the X-ray-emitting gas, of between similar to4% and 40%, consistent with an upper limit of similar to 40%, based directly on the observed limb-brightened morphology of the outflow. Only less than or similar to 20% of the observed X-ray emission can come from the volume-filling. metal-enriched, wind fluid itself. Spatially resolved spectroscopy of the soft diffuse thermal X-ray emission reveals that the predominant source of spectral variation along the outflow cones is due to strong variation in the absorption on scales of similar to 60 pc, there being little change in the characteristic temperature of the emission. We show that these observations are easily explained by, and fully consistent with, the standard model of a superwind driven by a starburst of the observed power of NGC 253. If these results are typical of all starburst-driven winds, then we do not directly see all the energy and gas tin particular the hot metal-enriched gas) transported out of galaxies by superwinds, even in X-ray emission.