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.Comment: 6 pages, Latex file, 2 figures, mn.sty, MNRAS in pres
