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 g alaxy NGC 253 with Chandra reveals a well-collimated, strongly limb-brighte ned, kiloparsec-scale conical outflow from the central starburst region. Th e 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 fir st time, robust evidence that both X-ray and H alpha emission come from low volume filling factor regions of interaction between the fast energetic wi nd of SN ejecta and the denser ambient interstellar medium and not from the wind fluid itself. We provide estimates of the (observationally and theore tically important) filling factor of the X-ray-emitting gas, of between sim ilar to4% and 40%, consistent with an upper limit of similar to 40%, based directly on the observed limb-brightened morphology of the outflow. Only le ss than or similar to 20% of the observed X-ray emission can come from the volume-filling. metal-enriched, wind fluid itself. Spatially resolved spect roscopy of the soft diffuse thermal X-ray emission reveals that the predomi nant 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 litt le change in the characteristic temperature of the emission. We show that t hese observations are easily explained by, and fully consistent with, the s tandard 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 emis sion.