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

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

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.

Citation

S. Bolton, J., Peng Oh, S., & R. Furlanetto, S. (2009). The evolution of the Lyman-alpha forest effective optical depth following HeII reionisation. Monthly Notices of the Royal Astronomical Society, 396(4), 2405–2418. https://doi.org/10.1111/j.1365-2966.2009.14914.x

• Journal Article Type: Article
• Acceptance Date: Apr 15, 2009
• Online Publication Date: Jul 1, 2009
• Publication Date: 2009-07
• Deposit Date: Sep 16, 2019
• Publicly Available Date: Nov 3, 2019
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Oxford University Press
• Peer Reviewed: Peer Reviewed
• Volume: 396
• Issue: 4
• Pages: 2405–2418
• DOI: https://doi.org/10.1111/j.1365-2966.2009.14914.x
• Keywords: Cosmology and Nongalactic Astrophysics
• Public URL: https://nottingham-repository.worktribe.com/output/2462301
• Publisher URL: https://academic.oup.com/mnras/article/396/4/2405/1031281
• Additional Information: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2009 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
• Contract Date: Nov 3, 2019