The Galactic Halo’s OviResonance Line Intensity

We used FUSE to observe ultraviolet emission from diffuse O VI in the hot gasin the Galactic halo. By comparing our result with another, nearby observationblocked by an opaque cloud at a distance of 230 pc, we could subtract off thecontribution from the Local Bubble, leading to an apparent halo intensity ofI_{OVI} = 4680^{+570}_{-660} photons/cm^2/s/sr. A correction for foregroundextinction leads to an intrinsic intensity that could be as much as twice thisvalue. Assuming T ~ 3 x 10^5 K, we conclude that the electron density, n_e, is0.01-0.02 /cm^3, the thermal pressure, p/k, is 7000-10,000 K/cm^3, and that thehot gas is spread over a length of 50-70 pc, implying a small filling factorfor O VI-rich gas. ROSAT observations of emission at 1/4 keV in the samedirection indicate that the X-rays are weaker by a factor of 1.1 to 4.7,depending on the foreground extinction. Simulated supernova remnants evolvingin low density gas have similar O VI to X-ray ratios when the remnant plasma isapproaching collisional ioinizational equilibrium and the physical structuresare approaching dynamical ``middle age''. Alternatively, the plasma can bedescribed by a temperature power-law. Assuming that the material isapproximately isobaric and the length scales according to T^(beta) d(ln T), wefind beta = 1.5+/-0.6 and an upper temperature cutoff of 10^{6.6(+0.3,-0.2)} K.The radiative cooling rate for the hot gas, including that which is too hot tohold O VI, is 6 x 10^{38} erg/s/kpc^2. This rate implies that ~70% of theenergy produced in the disk and halo by SN and pre-SN winds is radiated by thehot gas in the halo.Comment: Accepted by the Astrophysical Journal, AASTEX, 39 pages, including 6 figures and 4 tables (manuscript replaced in order to re-typeset Table 3 and correct submission entry