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

Abstract

We present arcsecond resolution Chandra X-ray and ground-based optical H-alpha imaging of a sample of ten edge-on star-forming disk galaxies (seven starburst and three ``normal'' spiral galaxies), a sample which covers the full range of star-formation intensity found in disk galaxies. We use the unprecedented spatial resolution of the Chandra X-ray observatory to robustly remove point sources, and hence obtain the X-ray properties of the diffuse thermal emission alone. The X-ray observations are combined with comparable-resolution H-alpha and R-band imaging, and presented as a mini-atlas of images on a common spatial and surface brightness scale. The vertical distribution of the halo-region X-ray surface brightness is best described as an 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 galaxies can be fit with a common two-temperature spectral model with an enhanced alpha-to-iron element ratio. This is consistent with the origin of the X-ray emitting gas being either metal-enriched merged SN ejecta or shock-heated ambient halo or disk material with moderate levels of metal depletion onto dust. The thermal X-ray emission observed in the halos of the starburst galaxies is either this pre-existing halo medium, which has been swept-up and shock heated by the starburst-driven wind, or wind material compressed near the walls of the outflow by reverse shocks within the wind. In either case the X-ray emission provides us with a powerful probe of the properties of gaseous halos around star-forming disk galaxies.