Impact of Helium II Resonant Absorption on the Ultraviolet Background Modeled in Three Dimensions

We implement a treatment of helium II absorption and reemission into the TECHNICOLOR DAWN cosmological simulations to study its impact on the metagalactic ultraviolet background (UVB) in three dimensions. By comparing simulations with and without He II reprocessing, we show that it weakens the mean UVB by ∼3 dex from z = 10–5 between 3.5 and 4 Ryd, where the He II Lyman series resonance occurs. In overdense regions, the overall UVB amplitude is higher, and the impact of He II reprocessing is weaker, qualitatively indicating an early start to He II reionization near galaxies. Comparing our simulations to two popular one-dimensional UV models, we find good agreement up to 3 Ryd at z = 5. At higher energies, our simulation shows significantly greater He II absorption because it accounts for He II arising both in diffuse regions and in Lyman limit systems. By contrast, the comparison models account only for He II in Lyman limit systems, which are subdominant prior to the completion of He II reionization. The H I and He II reionization histories are nearly unaffected by He II reprocessing, although the cosmic star formation rate density is altered by up to 4%. The cosmic mass density of C IV is reduced by ∼2 dex when He II is accounted for, while Si IV, C II, Mg II, Si II, and O I are unaffected.