W. Van Dyke Dixon and Arthur F. Davidsen
Department of Physics and Astronomy,
The Johns Hopkins University
Charles and 34th Streets,
Baltimore, Maryland 21218
wvd@pha.jhu.edu, afd@pha.jhu.edu
and
Henry C. Ferguson
Space Telescope Science Institute,
Baltimore, Maryland 21218
ferguson@stsci.edu
To appear in The Astrophysical Journal
and 88
in spectra obtained by the Hopkins Ultraviolet Telescope
(HUT) during the Astro-2 space shuttle mission in 1995 March. Of four
spectra taken through a 19'' 
197'' aperture, two show O VI 
emission at a significance of 4 
. Two out of six spectra taken
through a 10'' 
56'' aperture also show O VI emission, though at only
2 
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,
b = 56, may represent a more
typical region of the halo. In its spectrum, we find I(O VI
ergs cm
s
. This is the first detection of
OVI emission from the Galactic halo. None of the spectra exhibit
significant emission from CIV 
, though our upper limits are greater
than the intensities reported for other lines of sight. We set a
limit on I(O VI
C IV
, consistent with the
predictions of self-photoionizing galactic fountain models, but higher
than those of models based on turbulent mixing layers. Combining our
measured OVI intensity with estimates of 
OVI ) through the
halo, we find that, for 
, the data are
consistent with 
cm
and 
cm
K, values substantially greater than those derived from
CIV observations, suggesting that the CIV and OVI emission
arise from physically distinct clouds and/or that a substantial portion
of the CIV absorption arises from cooler gas that does not
contribute to the CIV emission. This result is consistent with
galactic halo models incorporating self-photoionization of the cooling
gas.
Subject headings: Galaxy: halo --- ultraviolet: ISM