The evolution of the Lyman-alpha forest effective optical depth following HeII reionisation

TL;DR

The paper investigates the cause of a narrow dip in the Lyman-alpha forest optical depth at z=3.2, concluding it is likely due to a peak in hydrogen ionisation rate rather than rapid IGM heating during HeII reionisation.

Contribution

It challenges previous interpretations by showing that the observed feature is better explained by a transient increase in hydrogen ionisation rate, not rapid heating during HeII reionisation.

Findings

A rapid temperature boost in the IGM does not produce the observed narrow dip.

A peak in hydrogen photo-ionisation rate explains the narrow feature.

Extended HeII reionisation leads to smooth optical depth evolution.

Abstract

Three independent observational studies have now detected a narrow (\Delta z ~ 0.5) dip centred at z=3.2 in the otherwise smooth redshift evolution of the Lya 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 HeII reionisation. We examine this interpretation using a semi-analytic model of inhomogeneous HeII reionisation and high resolution hydrodynamical simulations of the Lya forest. We instead find that a rapid (\Delta z ~ 0.2) boost to the IGM temperature (\Delta T ~ 10^4 K) beginning at z=3.4 produces a well understood and generic evolution in the Lya 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 HeII reionisation is instead extended over several redshift units, as recent theoretical studies suggest, then the Lya opacity will evolve smoothly with redshift. We conclude that the sharp dip observed in the Lya 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 HeII reionisation, or the opacity of Lyman limit systems.