Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...454l..23f&link_type=abstract
Astrophysical Journal Letters v.454, p.L23
Astronomy and Astrophysics
Astronomy
4
Galaxies: Halos, Galaxies: Individual Ngc Number: Ngc 4631, Galaxies: Spiral, Ultraviolet: Galaxies
Scientific paper
During the Astro-2 mission in 1995 March, 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 H alpha 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 6 x 105 K. Gas cooling through this temperature range should show emission in the prominent O VI lambda lambda 1032, 1038 and C IV lambda lambda 1548, 1551 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 C II lambda 1335 and O III] lambda lambda 1661, 1666. The aperture of the HUT spectrograph subtends 10" x 56" and was positioned parallel to the disk 39" (1.4 kpc) south of the nucleus. No emission lines were detected. The derived constraints are tempered somewhat 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 105 K or that mixing layers are not a significant contributor to the observed H alpha 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&sun; yr-1 are processed through a galactic fountain in NGC 4631.
Davidsen Arthur F.
Dettmar Ralf-Juergen
Dixon William Van Dyke
Ferguson Henry C.
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