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 ofstars. However, the derivation of accurate Si/C abundances requires reliableionization corrections, which are strongly dependent on the spectral shape ofthe metagalactic ionizing background and on the ``local effects'' of hot starsin nearby galaxies. Recent models have assumed power-law quasar ionizingbackgrounds plus a decrement at 4 Ryd to account for He II attenuation inintervening clouds. However, we show that realistic ionizing backgrounds basedon cosmological radiative transfer models produce more complex ionizing spectrabetween 1-5 Ryd that are critical to interpreting ions of Si and C. We alsomake a preliminary investigation of the effects of He II ionization frontnon-overlap. Because the attenuation and re-emission by intervening cloudsenhance Si IV relative to C IV, the observed high Si IV/C IV ratios do notrequire an unrealistic Si overproduction [Si/C $\geq 3 (Si/C)_{\odot}$]. If theionizing spectrum is dominated by ``local effects'' from massive stars, evenlarger Si IV/C IV ratios are possible. However, unless stellar radiationdominates quasars by more than a factor of 10, we confirm the evidence for someSi overproduction by massive stars; values Si/C $\approx 2 (Si/C)_{\odot}$ fitthe measurements better than solar abundances. Ultimately, an adequateinterpretation of the ratios of C IV, Si IV, and C II may require hot,collisionally ionized gas in a multiphase medium.Comment: 18 pages, latex, 8 figures, postscript; accepted by Astronomical Journal, to appear in vol. 113, no. 5 (May, 1997