COSMIC METAL PRODUCTION AND THE CONTRIBUTION OF QSO ABSORPTION SYSTEMS TO THE IONIZING BACKGROUND

The recent discovery by Cowie et al. (1995) and Tytler et al. (1995) o f metals in the Ly alpha clouds shows that the intergalactic medium (I GM) at high redshift is contaminated by the products of stars and sugg ests that ionizing photons from massive star formation may be a signif icant contributor to the UV background radiation at early epochs. We a ssess the validity of the stellar photoionization hypothesis. Based on recent computations of metal yields and O-star Lyman continuum (Lyc) fluxes, we find that 0.2% of the rest-mass energy of the metals produc ed is radiated as Lye. By modeling the transfer of ionizing radiation through the IGM and the rate of chemical enrichment, we demonstrate th at the background intensity of photons at 1 ryd that accompanies the p roduction of metals in the Ly alpha forest clouds may be significant, approaching 0.5 x 10(-21) ergs cm(-2) s(-1) Hz(-1) sr(-1) at z approxi mate to 3 if the Lyc escape fraction is greater than or similar to 0.2 5. Together with quasars, massive stars could then, in principle, prov ide the hydrogen and helium Lye photons required to ionize the univers e at high redshifts. We propose that observations of the He II Gunn-Pe terson effect and of the metal ionization states of the Lye forest and Lyman-limit absorbers should show the signature of a stellar spectrum . We also note that the stellar photoionization model fails if a large fraction of the UV radiation emitted from stars cannot escape into th e IGM, as suggested by the recent Hopkins Ultraviolet Telescope observ ations by Leitherer et al. (1995) of low-redshift starburst galaxies, or if most of the metals observed at z x 3 were produced at much earli er epochs.