THE STELLAR UV BACKGROUND AT Z-LESS-THAN-1.5 AND THE BARYON DENSITY OF PHOTOIONIZED GAS

We use new studies of the cosmic evolution of star-forming galaxies to estimate the production rate of ionizing photons from hot, massive st ars at low and intermediate redshifts. The luminosity function of blue galaxies in the Canada-France Redshift Survey shows appreciable evolu tion in the redshift interval z = 0-1.3, and generates a background in tensity at 1 Ryd of J(L) approximate to 1.3 x 10(-21) [f(esc)] erg cm( -2) s(-1) Hz(-1) sr(-1) at z approximate to 0.5, where [f(esc)] is the unknown fraction of stellar Lyman continuum photons that can escape i nto intergalactic space, and we have assumed that the absorption is pi cket fence type. We argue that recent upper Limits on the Hot surface brightness of nearby intergalactic clouds constrain this fraction to b e less than or similar to 20 per cent. The background ionizing flux fr om galaxies can exceed the QSO contribution at z approximate to 0.5 if [f(esc)] > 6 per cent. We show that, in the general framework of a di ffuse background dominated by QSOs and/or star-forming galaxies, the c osmological baryon density associated with photoionized, optically thi n gas decreases rapidly with cosmic time. The results of a recent Hubb le Space Telescope survey of O VI absorption lines in QSO spectra sugg est that most of this evolution may be due to the bulk heating and col lisional ionization of the intergalactic medium by supernova events in young galaxy haloes.