The Stellar UV Background at z<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 stars at low and intermediate redshifts. The luminosity function of blue galaxies in the Canada-France Redshift Survey shows appreciable evolution in the redshift interval z=0-1.3, and generates a background intensity at 1 ryd of J_L~ 1.3 x 10^{-21} f_{esc} ergs cm^{-2} s^{-1} Hz^{-1} sr^{-1} at z~0.5, where f_esc is the unknown fraction of stellar Lyman-continuum photons which can escape into the intergalactic space, and we have assumed that the absorption is picket fence-type. We argue that recent upper limits on the H-alpha surface brightness of nearby intergalactic clouds constrain this fraction to be <~ 20%. The background ionizing flux from galaxies can exceed the QSO contribution at z~ 0.5 if f_{esc}>~ 6%. We show that, in the general framework of a diffuse background dominated by QSOs and/or star-forming galaxies, the cosmological baryon density associated with photoionized, optically thin gas decreases rapidly with cosmic time. The results of a recent Hubble Space Telescope survey of OVI absorption lines in QSO spectra suggest that most of this evolution may be due to the bulk heating and collisional ionization of the intergalactic medium by supernova events in young galaxy halos.