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 Iimg1.gif = 4680img2.gif photons cm-2 s-1 sr-1. A correction for foreground extinction leads to an intrinsic intensity that could be as much as twice this value. Assuming T ~ 3 x 105 K, we conclude that the electron density, ne, is 0.01-0.02 cm-3, that the thermal pressure, p/k, is 7000-10,000 cm-3 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 1.1-4.7, 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 ionizational 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 Tbd ln T, we find b = 1.5 +- 0.6 and an upper temperature cutoff of 106.6(+0.3,-0.2) K. The radiative cooling rate for the hot gas, including that which is too hot to hold O VI, is 6 x 1038 ergs s-1 kpc-2. 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.