The Hot Gas Surrounding the Edge‐on Galaxy NGC 891

The previous discovery of an X-ray halo around the edge-on galaxy NGC 891 prompted this effort to better understand the structure of the hot halo by deep observations with the X-ray satellite telescopes ROSAT and ASCA. The ROSAT HRI image (FWHM of 5'') reveals four point sources in the disk and one in the halo that was confused with diffuse emission in the previous ROSAT PSPC image. At 5'' angular resolution, the halo emission remains diffuse, rather than becoming an ensemble of filaments and point sources. There is an indication of X-ray absorption in the midplane on the same angular scale as the optical extinction by dust (a width of 10''-20''). The PSPC observations (FWHM of 25''-30'') show that the diffuse emission along the disk closely follows the tracers of star formation, such as CO line and 1.3 mm continuum observations. This would be expected if massive stars led to the production of hot gas, such as through supernovae. The maximum vertical extent of the hot gas, ≈ 2', is similar to that of optical emission line gas, except the X-ray scale height is much larger. The halo gas density can be fit with the functional form where n 0 = 0.026 cm -3 , the radial length scale h = 85'' (4.1 kpc), and the vertical scale height l = 72 4 (3.5 kpc). The mean temperature of the gaseous component is 3.5 × 10 6 K, so for a filling factor of unity, the characteristic pressure is p / k ~ 10 5 K cm -3 , comparable to that of the molecular gas in the disk. The total gas mass is 4.0 × 10 7 M ☉ , the bolometric X-ray luminosity is 3.9 × 10 39 ergs s -1 , and the mass cooling rate is 0.087 M ☉ yr -1 .