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 a nd 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 emissio n in the previous ROSAT PSPC image. At 5'' angular resolution, the hal o emission remains diffuse, rather than becoming an ensemble of filame nts and point sources. There is an indication of X-ray absorption in t he midplane on the same angular scale as the optical extinction by dus t (a width of 10''-20''). The PSPC observations (FWHM of 25''-30'') sh ow that the diffuse emission along the disk closely follows the tracer s 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 h ot gas, approximate to 2', is similar to that of optical emission line gas, except the X-ray scale height is much larger. The halo gas densi ty can be fit with the functional form n(e)(x, z) = n(o) exp (-x/h) ex p (-z(2)/l(2)), 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). Th e mean temperature of the gaseous component is 3.5 x 10(6) K, so for a filing factor of unity, the characteristic pressure is p/k similar to 10(5) K cm(-3), comparable to that of the molecular gas in the disk. The total gas mass is 4.0 x 10(7) M-circle dot, the bolometric X-ray l uminosity is 3.9 x 10(39) ergs s(-1), and the mass cooling rate is 0.0 87 M-circle dot yr(-1).