A SEARCH FOR FAR-ULTRAVIOLET EMISSION-LINES FROM DIFFUSE HOT GAS IN THE HALO OF NGC-4631

During the Astro-2 mission in 1995 March, the Hopkins Ultraviolet Tele scope (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 ra dio continuum emission, extended H(Y emission, and soft X-ray emission from an extended halo. Recent ROSAT observations indicate that much o f the X-ray emission comes from a very soft component, with a temperat ure less than 6 x 10(5) K. Gas cooling through this temperature range should show emission in the prominent O VI lambda lambda 1032, 1038 an d C IV lambda lambda 1548, 1551 lines. Hot gas interacting with cold c louds at the interface of the disk and the halo may form turbulent mix ing layers, which should produce strong emission in lower ionization s pecies such as C II lambda 1335 and O III] lambda lambda 1661, 1666. T he aperture of the HUT spectrograph subtends 10 '' x 56 '' and was pos itioned parallel to the disk 39 '' (1.4 kpc) south of the nucleus. No emission lines were detected. The derived constraints are tempered som ewhat by the unknown extinction within the NGC 4631 halo. If it is low , the upper limits suggest that the mean mixing-layer temperatures are less than 2 x 10(5) K or that mixing layers are not a significant con tributor to the observed Ha emission at the position of the HUT slit. For smooth flows, the upper limit to the O VI flux suggests that less than approximately 15 M. yr(-1) are processed through a galactic fount ain in NGC 4631.