ar X iv : a st ro - p h / 06 12 47 6 v 3 2 9 D ec 2 00 6 The Galactic Halo ’ s O VI Resonance Line Intensity

We used FUSE to observe ultraviolet emission from diffuse O VI in the hot gas in the Galactic halo. By comparing our result with another, nearby observation blocked by an opaque cloud at a distance of 230 pc, we could subtract off the contribution from the Local Bubble, leading to an apparent halo intensity of IOV I = 4680 −660 photons cm −2 s sr. A correction for foreground extinction leads to an intrinsic intensity that could be as much as twice this value. Assuming T ∼ 3× 10 K, we conclude that the electron density, ne, is 0.01−0.02 cm, the thermal pressure, p/k, is 7000−10, 000 cm K, and that the hot gas is spread over a length of 50-70 pc, implying a small filling factor for O VI-rich gas. ROSAT observations of emission at 1/4 keV in the same direction indicate that the X-rays are weaker by a factor of 1.1 to 4.7, depending on the foreground extinction. Simulated supernova remnants evolving in low density gas have similar O VI to X-ray ratios when the remnant plasma is approaching collisional ioinizational equilibrium and the physical structures are approaching dynamical “middle age”. Alternatively, the plasma can be described by a temperature power-law. Assuming that the material is approximately isobaric and the length scales according to T d lnT , we find β = 1.5± 0.6 and an upper temperature cutoff of 10 K. The radiative cooling rate for the hot gas, including that which is too hot to hold O VI, is 6× 10 erg s−1kpc. This rate implies that ∼70% of the energy produced in the disk and halo by SN and pre-SN winds is radiated by the hot gas in the halo.