A Search for Far-UV Emission Lines from Diffuse Hot Gas in the Halo of NGC4631
Henry C. Ferguson
Space Telescope Science Institute, Baltimore, MD 21218
ferguson@stsci.edu
W. Van Dyke Dixon, Arthur F. Davidsen
Department of Physics and Astronomy, Johns Hopkins University
Charles and 34th Streets, Baltimore, Maryland 21218
wvd@pha.jhu.edu, afd@pha.jhu.edu
Ralf-Juergen Dettmar
Astronomisches Institut, Ruhr-Universitaet Bochum,
Universitätsstrasse 150/ NA 7, D-44780 BOCHUM, F. R. GERMANY
dettmar@wsa.astro.ruhr-uni-bochum.de
Abstract:
During the Astro-2 mission in March of 1995, the Hopkins Ultraviolet
Telescope (HUT) was used to conduct a sensitive search for far-UV
emission lines from hot gas in the halo of NGC 4631. This galaxy has
strong radio continuum emission, extended 
emission, and
soft X-ray emission from an extended halo. Recent ROSAT observations
indicate that much of the X-ray emission comes from a very soft
component, with a temperature less than 
K. Gas cooling
through this temperature range should show emission in the prominent
OVI 
and CIV 
lines. Hot gas interacting with cold clouds at the
interface of the disk and the halo may form turbulent mixing layers,
which should produce strong emission in lower-ionization species such
as CII 
and OIII] 
.
The aperture of the HUT spectrograph subtends 
and was positioned parallel to the disk
(1.4 kpc) south of the nucleus. No emission lines
were detected. The constraints are tempered somewhat by the unknown
extinction within the NGC4631 halo. If this is low, the upper limits
suggest that the mean mixing-layer temperatures are less than 
K or that mixing layers are not a significant contributor to the
observed H
emission at the position of the HUT slit. For smooth flows,
the upper limit to the OVI flux suggests that less than
are processed through a galactic
fountain in NGC4631.
ferguson@stsci.edu