DISCOVERY OF O VI EMISSION FROM THE GALACTIC CORONA WITH THE HOPKINS ULTRAVIOLET TELESCOPE

We have searched for far-UV emission from coronal gas in the Galactic halo along lines of sight with Galactic latitudes between 42 degrees a nd 88 degrees in spectra obtained by the Hopkins Ultraviolet Telescope during the Astro-2 space shuttle mission in 1995 March. Of four spect ra taken through a 19 '' x 197 '' aperture, two show O VI lambda lambd a 1032, 1038 emission at a significance of 4 sigma. Two out of six spe ctra taken through a 10 '' x 56 '' aperture also show O VI emission, t hough at only 2 sigma 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 degrees, b = 56 degrees, 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 detectio n of O VI emission from the Galactic halo. None of the spectra exhibit significant emission from C IV lambda lambda 1548, 1551, though our u pper limits are greater than the intensities reported for other lines of sight. We set a limit on I(O VI)/I(C IV) greater than or equal to 3 .4, consistent with the predictions of self-photoionizing Galactic fou ntain models, but higher than those of models based on turbulent mixin g layers. Combining our measured O VI intensity with estimates of N(O VI) through the halo, we find that, for 5.3 less than or equal to log T less than or equal to 5.8, the data are consistent with n(e) approxi mate to 0.06 cm(-3) and 22,000 less than or equal to P/k less than or equal to 67,000 cm(-3) K, values substantially greater than those deri ved from C IV observations, suggesting that the C IV and O VI emission arise from physically distinct clouds and/or that a substantial porti on of the C IV absorption arises from cooler gas that does not contrib ute to the C IV emission. This result is consistent with Galactic halo models incorporating self-photoionization of the cooling gas.