The influence of the photoionizing radiation spectrum on metal-line ratios in LY alpha forest clouds

Recent measurements of Si IV/C IV ratios in the high-redshift Ly alpha forest [Songaila & Cowie, AJ, 112, 335 (1996a); Savaglio et al., A&A (in press) (1997)] have opened a new window on chemical enrichment and the first generations of stars. However, the derivation of accurate Si/C abundances requires reliable ionization corrections, which are strongly dependent on the spectral shape of the metagalactic ionizing background and on the ''local effects'' of hot stars in nearby galaxies. Recent models have assumed power-law quasar ionizing backgrounds plus a decrement at 4 Ryd to account for He II attenuation in intervening clouds. However, we show that realistic ionizing backgrounds based on cosmological radiative transfer models produce more complex ionizing spectra between 1-5 Ryd that are critical to interpreting ions of Si and C. We also make a preliminary investigation of the effects of He II ionization front nonoverlap. Because the attenuation and reemission by intervening clouds enhance Si rv relative to C IV, the observed high Si IV/C rv ratios do not require an unrealistic Si overproduction [Si/C greater than or equal to 3 (Si/C).]. If the ionizing spectrum is dominated by ''local effects'' from massive stars, even larger Si IV/C Iv ratios are possible. However, unless stellar radiation dominates quasars by more than a factor of 10, we confirm the evidence for some Si overproduction by massive stars; values Si/C approximate to 2 (Si/C). fit the measurements better than solar abundances. Ultimately, an adequate interpretation of the ratios of C Iv, Si IV, and C II may require hot, collisionally ionized gas in a multiphase medium. (C) 1997 American Astronomical Society.