The evolution of the Lyα forest effective optical depth following He ii reionization

Three independent observational studies have now detected a narrow (delta z similar or equal to 0.5) dip centred at z = 3.2 in the otherwise smooth redshift evolution of the Ly alpha forest effective optical depth. This feature has previously been interpreted as an indirect signature of rapid photoheating in the intergalactic medium (IGM) during the epoch of He ii reionization. We examine this interpretation using a semi-analytic model of inhomogeneous He ii reionization and high-resolution hydrodynamical simulations of the Ly alpha forest. We instead find that a rapid (delta z similar or equal to 0.2) boost to the IGM temperature (delta T similar or equal to 10(4) K) beginning at z = 3.4 produces a well understood and generic evolution in the Ly alpha effective optical depth, where a sudden reduction in the opacity is followed by a gradual, monotonic recovery driven largely by adiabatic cooling in the low-density IGM. This behaviour is inconsistent with the narrow feature in the observational data. If photoheating during He ii reionization is instead extended over several redshift units, as recent theoretical studies suggest, then the Ly alpha opacity will evolve smoothly with redshift. We conclude that the sharp dip observed in the Ly alpha forest effective optical depth is instead most likely due to a narrow peak in the hydrogen photoionization rate around z = 3.2, and suggest that it may arise from the modulation of either reprocessed radiation during He ii reionization, or the opacity of Lyman limit systems.