The Redshift Evolution of the Metagalactic Ionizing Flux Inferred from Metal Line Ratios in the Lyman Forest

Metal line ratios in a sample of 13 quasar spectra obtained with the HIRES spectrograph on the Keck I Telescope have been analyzed to characterize the evolution of the metagalactic ionizing flux near a redshift of 3. The evolution of Si IV/C IV has been determined using three different techniques: using total column densities of absorption-line complexes, as in the approach of Songaila & Cowie, using the column densities of individual Voigt profile components within complexes, and using direct optical depth ratios. All three methods show that Si IV/C IV changes abruptly at z ∼ 3, requiring a jump in value of about a factor of 3.4 and indicating a significant change in the ionizing spectrum that occurs rapidly between z = 2.9 and z = 3, just above the redshift at which Reimers et al. detected patchy He II Lyα absorption. At lower redshifts, the ionization balance is consistent with a pure power-law ionizing spectrum, but at higher redshifts the spectrum must be very soft, with a large break at the He+ edge. An optical depth ratio technique is used to measure the abundances of ions whose transitions lie within the forest, and C III, Si III, and O VI are detected in this way. The presence of a significant amount of O VI at z > 3 suggests either a considerable volume of He III bubbles embedded in the more general region in which the ionizing flux is heavily broken or the addition of collisional ionization to the simple photoionization models.