The evolution of the Ly α forest effective optical depth following He II reionisation

Three independent observational studies have now detected a narrow (∆z ≃ 0.5) dip centred at z = 3.2 in the otherwise smooth redshift evolution of the Lyα forest effective optical depth. This feature has previously been interpreted as an indirect signature of rapid photo-heating in the IGM during the epoch of He II reionisation. We examine this interpretation using a semi-analytic model of inhomogeneous He II reionisation and high resolution hydrodynamical simulations of the Lyα forest. We instead find that a rapid (∆z ≃ 0.2) boost to the IGM temperature (∆T ≃ 10 K) beginning at z = 3.4 produces a well understood and generic evolution in the Lyα 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 photo-heating during He II reionisation is instead extended over several redshift units, as recent theoretical studies suggest, then the Lyα opacity will evolve smoothly with redshift. We conclude that the sharp dip observed in the Lyα forest effective optical depth is instead most likely due to a narrow peak in the hydrogen photo-ionisation rate around z = 3.2, and suggest that it may arise from the modulation of either reprocessed radiation during He II reionisation, or the opacity of Lyman limit systems.