Origin of the UV Background
Well, I have now (2009 August 30) finished a small investigation.
We've all seen that we ALWAYS get background radiation (as we call it, anyway) with GALEX.
Long ago, Professor Murthy and I came to the conclusion that there is a universal background, longward of about Lyman alpha, and that this background is NOT present shortward of about Lyman alpha (our Voyager work).
I decided to test our claim further, by looking at a much larger number of GALEX images - some from all over the sky, or rather the parts of the sky that have been observed with the GALEX all-sky survey (AIS).
There are 28,000 AIS images. I decided to measure the average sky background on as many as possible of the Voyager targets. We observed 430 Voyager targets; for 380 of them (I found) GALEX background could be measured.
Look at Fig.1, which shows GALEX brightness (but with 300 photon units subtracted from each measurement plotted against the Voyager observations. Note the excellent correlation. The dashed circles are the error value, for cases where we only got an upper limit. Now let's NOT subtract off our 300 photon unit background (see Fig. 2.).
This is even MORE gratifying. It seems abundantly clear that we have been exactly right all along: there is an essentially uniform background of 300 units! And of course in many directions, there is additional radiation (presumably always starlight scattered from dust). Log-base-10 of 300 is 2.5 which is the floor in the above diagram!
The result is very gratifying and informative. Dick.
In my opinion, the uniform background of 300 units seen as the floor in the GALEX data is most probably the AIRGLOW---Sujatha