Cosmic Metal Production and the Contribution of QSO Absorption Systems to the Ionizing Background

Abstract

The recent discovery by Cowie \etal and Tytler \etal of metals in the \Lya clouds shows that the intergalactic medium (IGM) at high redshift is contaminated by the products of stars, and suggests that ionizing photons from massive star formation may be a significant contributor to the UV background radiation at early epochs. We assess 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 produced is radiated as LyC. By modeling the transfer of ionizing radiation through the IGM and the rate of chemical enrichment, we demonstrate that the background intensity of photons at 1 Ryd that accompanies the production of metals in the \Lya forest clouds may be significant, approaching $0.5\times 10^{-21}\uvunits$ at $z\approx 3$ if the LyC escape fraction is $\gta 0.25$. Together with quasars, massive stars could then, in principle, provide the hydrogen and helium LyC photons required to ionize the universe at high redshifts. We propose that observations of the \HeII Gunn-Peterson effect and of the metal ionization states of the \Lya 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 the IGM, as suggested by the recent {\it Hopkins Ultraviolet Telescope} observations by Leitherer \etal of low-redshift starburst galaxies, or if most of the metals observed at $z\approx 3$ were produced at much earlier epochs.