VACUUM ULTRAVIOLET IMAGERY OF THE VIRGO CLUSTER REGION .3. DIFFUSE FAR-ULTRAVIOLET RADIATION AT HIGH GALACTIC LATITUDES

Far-ultraviolet radiation over a wide sky region of -110-degrees < l < 60-degrees and b > 60-degrees was observed by a rocket-borne two-dimensional imager in the wavelength around 150 nm. The background far-ultraviolet radiation from the Virgo cluster region was also observed in the pointing phase of the same rocket experiment. Contributions from several components, such as airglows, faint stars, or molecular hydrogen emission, to the observed radiation were examined, and the diffuse far-ultraviolet background radiation at high Galactic latitudes was derived. The results showed a steep dependence of the background radiation on the Galactic latitude b, and the radiation flux in the region of b > 70-degrees and of N(H I) < 3 x 10(20) cm-2 to be 300-600 photons cm-2 s-1 angstrom-1 sr-1. A clear correlation of the background radiation with the hydrogen column density N(H I) was obtained in the region of b > 70-degrees. The correlation was confirmed by the background radiation in the Virgo cluster region, indicating that major part of the background radiation is Galactic origin. The observed steep dependence on b cannot be explained easily by simple models of light scattering by interstellar dust grains with uniform source distribution. We propose a model of light scattering by dust with anisotropically distributed far-ultraviolet stellar sources in the Galactic disk. Comparison of the model calculation with the observation indicates that, as for scattering properties of the interstellar dust grains, the albedo a is larger than 0.32 and the scattering phase asymmetry factor g is greater than 0.5. These results are in agreement with some theoretical predictions. The regression line of the observed flux versus N(H I) intercepts with the ordinate axis at 200 to 300 photons cm-2 s-1 angstrom-1 sr-1, in which the Galactic isotropic component and the extragalactic component are involved. The difference in the background radiation between inside and outside the Virgo cluster was shown to be less than 2 photons cm-2 s-1 angstrom-1 sr-1. This can set an upper limit to the decay light of neutrinos from the Virgo cluster, and a lower limit of about 6 x 10(24) s to be lifetime of the heavy neutrinos of the mass around 15 eV c-2, if the mass of the Virgo cluster derived from the X-ray observations is allocated to heavy neutrinos.