Observations 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 arcdeg and 88 arcdeg using spectra obtained by the Hopkins Ultraviolet Telescope (HUT) during the Astro-2 space shuttle mission in 1995 March. Of four spectra taken through a Slit five aperture, two show Osixwave emission at a significance of 4 sigma . Two out of six spectra taken through a Slit-six aperture also show Osix emission, though 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 arcdeg, b = 56 arcdeg, may represent a more typical region of the halo. In its spectrum, we find I( Osix) = (3.59 +/- 0.96) x 10(-7) ERGS. This is the first detection of Osix emission from the Galactic halo. None of the spectra exhibit significant emission from Cfourwave, though our upper limits are greater than the reported intensities along other lines of sight. We set a limit on I( Osix) I( Cfour) >= 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 Osix intensity with estimates of N( Osix) 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 22000 <= P/k <= 67000 cm(-3) K, substantially greater than the values derived from Cfour observations, suggesting that the Cfour and Osix emission arise from physically distinct clouds and/or that a substantial portion of the Cfour absorption arises from nonemitting photoionized gas. This result is consistent with galactic halo models incorporating self-photoionization of the cooling gas.
- Publication:
-
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1995
- Bibcode:
- 1995AAS...18712004D