The Extragalactic Diffuse Background in the Far Ultraviolet
The Galactic and Extragalactic Background Radiation
Due mainly to the minimal contaminating effects of zodiacal light and direct stellar emission, the far UV wavelength band from 912 to = 2000 Â is ideally suited, in principle at least, for an accurate measurement of a diffuse background component due to sources outside our own galaxy. The cosmological significance of such radiation is of great current interest as it certainly includes the cumulative line-of-sight effect of galaxies and quasars and may include emission from both a lukewarm
... th a lukewarm intergalactic medium and decaying massive particles such as neutrinos, photinos, etc. The radiation required to maintain the IGM at its known high ionization level should also, in any case, appear clearly in this band due to redshift and lookback time effects, thereby providing a crucial clue as to its presendy obscure origins. Just how accurately this component can be measured in practice, however, clearly depends on how well we understand the probably dominant galactic component from which it must be disentangled in one way or another except, possibly, at the galactic poles. The residual emission there in this band is on the order of a few xlO 2 photons cm'VV" 1 A" 1 , of which perhaps as many as 50 units are almost certainly due to galaxies since the small-scale spatial fluctuations corresponding to this flux almost exacdy mimic those expected from the known spatial distribution of galaxies. The rest must come from a presendy uncertain source, most likely a residual tenuous dust layer at the galactic poles. This latter possibility is at least consistent with recent IRAS results on the diffuse IR background at 100 μπι and very sensitive HI, 21 cm measurements in these regions, but an extragalactic origin cannot be presendy ruled out Higher spatial and spectral resolution observations throughout the entire far UV range planned for the near future from orbiting platforms are expected to resolve this last but critically important issue. 307 S. Bowyer and C. Leinert (eds.), The Galactic and Extragalactic Background Radiation, 307-316.