A high spatial resolution X-ray and H-alpha study of hot gas in the halos of star-forming disk galaxies. I. Spatial and spectral properties of the diffuse X-ray emission

摘要

We present arcsecond resolution Chandra X-ray and ground-based opticalH-alpha imaging of a sample of ten edge-on star-forming disk galaxies (sevenstarburst and three ``normal'' spiral galaxies), a sample which covers the fullrange of star-formation intensity found in disk galaxies. We use theunprecedented spatial resolution of the Chandra X-ray observatory to robustlyremove point sources, and hence obtain the X-ray properties of the diffusethermal emission alone. The X-ray observations are combined withcomparable-resolution H-alpha and R-band imaging, and presented as a mini-atlasof images on a common spatial and surface brightness scale. The verticaldistribution of the halo-region X-ray surface brightness is best described asan exponential, with the observed scale heights lying in the range H_eff = 2 --4 kpc. The ACIS X-ray spectra of extra-planar emission from all these galaxiescan be fit with a common two-temperature spectral model with an enhancedalpha-to-iron element ratio. This is consistent with the origin of the X-rayemitting gas being either metal-enriched merged SN ejecta or shock-heatedambient halo or disk material with moderate levels of metal depletion ontodust. The thermal X-ray emission observed in the halos of the starburstgalaxies is either this pre-existing halo medium, which has been swept-up andshock heated by the starburst-driven wind, or wind material compressed near thewalls of the outflow by reverse shocks within the wind. In either case theX-ray emission provides us with a powerful probe of the properties of gaseoushalos around star-forming disk galaxies.