Session 15. Galaxies
Display, Tuesday, November 7, 2000, 8:00am-6:00pm, Bora Bora Ballroom

## [15.14] Chandra observations of NGC 253: New insights into the nature of starburst-driven superwinds

D.K. Strickland, T.M. Heckman (JHU), K.A. Weaver (GSFC), M. Dahlem (Sterrewacht Leiden)

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 \lesssim 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 (ISM), 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 ~4 and 40 per cent, consistent with an upper limit of ~40 per cent based directly on the observed limb-brightened morphology of the outflow. Only \lesssim 20 per cent of the observed X-ray emission can come from the volume-filling, metal-enriched, wind fluid itself.

We show that these observations are easily explained by, and fully consistent with, the standard model of a superwind driven by a starburst of NGC 253's observed power. This observation implies that X-ray observations of starbursts do not directly probe the energetic, metal-enriched, gas that drives the observed superwind outflows. We discuss the implications of these findings for the enrichment and heating of the IGM by starburst-driven winds.

This work was supported by NASA through grants LTSA NAG56400 and GO0-1008X.
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