Discovery of O VI Emission from the Galactic Corona with the Hopkins Ultraviolet Telescope

Abstract

We have searched for far-UV emission from coronal gas in the Galactic halo along lines of sight with Galactic latitudes between 42^deg^ and 88^deg^ in spectra obtained by the Hopkins Ultraviolet Telescope during the Astro-2 space shuttle mission in 1995 March. Of four spectra taken through a 19" x 197" aperture, two show O VI λλ1032, 1038 emission at a significance of 4 σ. Two out of six spectra taken through a 10" x 56" aperture also show O VI emission, though at only 2 σ levels of significance. Three of the detections lie near regions of enhanced soft X-ray emission associated with Radio Loop I. The fourth, at l = 218^deg^, b = 56^deg^, may represent a more typical region of the halo. In its spectrum, we find I(O VI) = (3.59 +/- 0.96) x 10^-7^ ergs cm^-2^ s^-1^. This is the first detection of O VI emission from the Galactic halo. None of the spectra exhibit significant emission from C IV λλ1548, 1551, though our upper limits are greater than the intensities reported for other lines of sight. We set a limit on I(O VI)/I(C IV) >= 3.4, consistent with the predictions of self-photoionizing Galactic fountain models, but higher than those of models based on turbulent mixing layers. Combining our measured O VI intensity with estimates of N(O VI) through the halo, we find that, for 5.3 <= log T <= 5.8, the data are consistent with n_e_ ~ 0.06 cm^-3^ and 22,000 <= P/k <= 67,000 cm^-3^ K, values substantially greater than those derived from C IV observations, suggesting that the C IV and O VI emission arise from physically distinct clouds and/or that a substantial portion of the C IV absorption arises from cooler gas that does not contribute to the C IV emission. This result is consistent with Galactic halo models incorporating self-photoionization of the cooling gas.