Summary: Mem. della Soc. Astron. Italian, 70, No.s 3/4 825
Multifrequency Behaviour of High Energy Cosmic Sources, Memorie della Societa Astronomica Italian,
eds. F. Giovannelli and L. Sabau-Graziati, 70, No.s 3/4, pp. 825-830 , 1999
THE DIFFUSE ULTRAVIOLET BACKGROUND RADIATION
RICHARD C. HENRY
The Johns Hopkins University, Baltimore, Maryland
ABSTRACT. I review the nature of the diffuse cosmic ultraviolet background radiation. The ultraviolet background
is the last frontier: all the other diffuse backgrounds have been examined, at least at some level. The ultraviolet
background has only begun to be explored; it offers rich promise of new astrophysical knowledge.
The study of diffuse backgrounds has played an important role in the recent history of astronomy.
From the microwave discovery of the 2.7 K background, to the soft X-ray detection of coronal gas,
to the diffuse H emission from warm interstellar gas in our galaxy, to the infrared mapping of
wisps of dust at high galactic latitudes, diffuse background astronomy has provided fundamental
insights into the nature of the universe. As the various regions of the electromagnetic spectrum
have been explored, their diffuse backgrounds have been found to arise from the widest possible
range of sources: from the local interstellar medium to the farthest reaches of the observable
universe; from the wrinkled echo of the Big Bang to the million degree plasma between the stars.
Over the past twenty years, it has become increasingly clear that one relatively narrow
portion of the spectrum, the far ultraviolet band from the Lyman edge at 912 Å to 2000 Å, has a