Metal Enrichment and Ionization Balance
in the Lyman Alpha Forest at z=3

Si IV/C IV versus redshift for clouds that are not known Lyman limit systems. The tick marks show the redshifts below which He+ edge breaks have been measured. The optical depth measured by Davidsen et al. at z=2.743 is much lower (tau=1) than that measured by Jakobsen et al. at z=3.286 (tau >>1.7), above the abrupt transition. This most likely arises from the change in ionizing spectrum from one at z>3.1 that has few photons above the the He+ edge to one with an unbroken power law at lower redshift. The abruptness of the transition favors a crossing of the He+ ionization boundary of the IGM as the reason, rather than the other possibility, namely the changeover form a galaxy-dominated ionizing flux at z>3.1 to a quasar-dominated flux at lower redshift.