CARBON IONIZATION STATES AND THE COSMIC FAR-UV BACKGROUND WITH He II ABSORPTION

We constrain the spectrum of the cosmic ultraviolet background radiation by fitting the observed abundance ratios of carbon ions at z ∼ 2-3 with those expected from different models of the background radiation. We use the recently calculated modulation of the background radiation between 3 and 4 Ryd due to resonant line absorption by intergalactic He II and determine the ratios of C III to C IV expected at these redshifts, as functions of metallicity, gas density, and temperature. Our analysis of the observed ratios shows that 'delayed reionization' models, which assume a large fraction of He II at z ∼ 3, are not favored by data. Our results suggest that He II reionization was inhomogeneous, consistent with the predictions from recent simulations.