ChandraObservations of NGC 253. II. On the Origin of Diffuse X‐Ray Emission in the Halos of Starburst Galaxies

We present a detailed case study of the diffuse X-ray and H-alpha emission inthe halo of NGC 253, a nearby edge-on starburst galaxy driving a galacticsuperwind. The arcsecond spatial resolution of the Chandra ACIS instrumentallows us to study the spatial and spectral properties of the diffuse X-rayemitting plasma with greatly superior spatial and spectral resolution comparedto previous X-ray instruments. We find statistically significant structurewithin the X-ray diffuse emission on angular scales down to ~10 arcsec (~130pc). There is no statistically significant evidence for any spatial variationin the spectral properties of the diffuse emission, over scales from ~400 pc to\~3 kpc. We show that the X-shaped soft X-ray morphology of the superwind,previously revealed by ROSAT, is matched by very similar X-shaped H-alphaemission, extending at least 8 kpc above the plane of the galaxy. In thenorthern halo the X-ray emission appears to lie slightly interior to theboundary marked by the H-alpha emission. The total 0.3-2.0 keV energy bandX-ray luminosity of the northern halo, L_X ~ 5e38 erg/s, is very similar to thehalo H-alpha luminosity of L_Ha ~ 4e38 erg/s, both of which are a smallfraction of the estimated wind energy injection rate of ~1e42 erg/s fromsupernovae in the starburst. We show that there are a variety of models thatcan simultaneously explain spatially-correlated X-ray and H-alpha emission inthe halos of starburst galaxies. These findings indicate that the physicalorigin of the X-ray-emitting million-degree plasma in superwinds is closelylinked to the presence of much cooler and denser T ~ 1e4 K gas, not only withinthe central kpc regions of starbursts, but also on ~10 kpc-scales within thehalos of these galaxies. (Abridged)