A High Spatial Resolution X‐Ray and Hα 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 Hα 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. The X-ray observations make use of the unprecedented spatial resolution of the Chandra X-ray observatory to more robustly than before remove X-ray emission from point sources, and hence obtain the X-ray properties of the diffuse thermal emission alone. We have combined the X-ray observations with existing, comparable-resolution, ground-based Hα and R-band imaging, and present a mini-atlas of images on a common spatial and surface brightness scale to aid cross-comparison. In general, the morphology of the extra-planar diffuse X-ray emission is very similar to the extra-planar Hα filaments and arcs, on both small and large scales (scales of 10's of pc and kiloparsecs respectively). The most spectacular cases of this are found in NGC 1482 (for which we provide the first published X-ray observation) and NGC 3079. We provide a variety of quantitative measures of how the spectral hardness and surface brightness of the diffuse X-ray emission varies with increasing height z above the plane of each galaxy. Of the eight galaxies in which diffuse Xray emitting halos are found (the starbursts and the normal spiral NGC 891), significant spatial variation in the spectral properties of the extra-planar emission (|z| ≥ 2 kpc) is only found in two cases: NGC 3628 and NGC 4631. In general, the vertical distribution of the halo-region X-ray surface brightness is best described as an exponential, with the observed scale heights of the sample galaxies lying in the range H eff ∼ 2 -4 kpc. The presence of extra-planar X-ray emission is alway associated with the presence of extra-planar optical line emission of similar vertical extent. No X-ray emission was detected from the halos of the two low mass normal spiral galaxies NGC 6503 and NGC 4244. AGN, where present, appear to play no role in powering or shaping the outflows from the the starburst galaxies in this sample. The Chandra 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 α-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. Our favored model is that SN feedback in the disks of star-forming galaxies create, via blow out and venting of hot gas from the disk, tenuous exponential atmospheres of density scale height H g ∼ 4 -8 kpc. The soft 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 pro...

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