Journal Article

The evolution of the Lyα forest effective optical depth following He <span class="smallCaps">ii</span> reionization

James S. Bolton, S. Peng Oh and Steven R. Furlanetto

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 396, issue 4, pages 2405-2418
Published in print July 2009 | ISSN: 0035-8711
Published online July 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.14914.x
The evolution of the Lyα forest effective optical depth following He ii reionization

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Three independent observational studies have now detected a narrow (Δz≃ 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 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 Lyα forest. We instead find that a rapid (Δz≃ 0.2) boost to the IGM temperature (ΔT≃ 104 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 photoheating during He ii reionization 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 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.

Keywords: methods: numerical; intergalactic medium; quasars: absorption lines; cosmology: theory; diffuse radiation

Journal Article.  12816 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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