The mean free path of ionizing photons at 5 < z < 6: evidence for rapid evolution near reionization

Becker, George D.; D’Aloisio, Anson; Christenson, Holly M.; Zhu, Yongda; Worseck, Gabor; Bolton, James S

Abstract

The mean free path of ionizing photons, λmfp, is a key factor in the photoionization of the intergalactic medium (IGM). At z ≳ 5, however, λmfp may be short enough that measurements towards QSOs are biased by the QSO proximity effect. We present new direct measurements of λmfp that address this bias and extend up to z ∼ 6 for the first time. Our measurements at z ∼ 5 are based on data from the Giant Gemini GMOS survey and new Keck LRIS observations of low-luminosity QSOs. At z ∼ 6 we use QSO spectra from Keck ESI and VLT X-Shooter. We measure λmfp=9.09+1.62−1.28 proper Mpc and 0.75+0.65−0.45 proper Mpc (68 per cent confidence) at z = 5.1 and 6.0, respectively. The results at z = 5.1 are consistent with existing measurements, suggesting that bias from the proximity effect is minor at this redshift. At z = 6.0, however, we find that neglecting the proximity effect biases the result high by a factor of two or more. Our measurement at z = 6.0 falls well below extrapolations from lower redshifts, indicating rapid evolution in λmfp over 5 < z < 6. This evolution disfavours models in which reionization ended early enough that the IGM had time to fully relax hydrodynamically by z = 6, but is qualitatively consistent with models wherein reionization completed at z = 6 or even significantly later. Our mean free path results are most consistent with late reionization models wherein the IGM is still 20 per cent neutral at z = 6, although our measurement at z = 6.0 is even lower than these models prefer.

Citation

Becker, G. D., D’Aloisio, A., Christenson, H. M., Zhu, Y., Worseck, G., & Bolton, J. S. (2021). The mean free path of ionizing photons at 5 &lt; z &lt; 6: evidence for rapid evolution near reionization. Monthly Notices of the Royal Astronomical Society, 508(2), 1853-1869. https://doi.org/10.1093/mnras/stab2696

• Journal Article Type: Article
• Acceptance Date: Sep 14, 2021
• Online Publication Date: Sep 21, 2021
• Publication Date: Oct 7, 2021
• Deposit Date: Sep 17, 2021
• Publicly Available Date: Oct 7, 2021
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Oxford University Press
• Peer Reviewed: Peer Reviewed
• Volume: 508
• Issue: 2
• Pages: 1853-1869
• DOI: https://doi.org/10.1093/mnras/stab2696
• Keywords: Space and Planetary Science; Astronomy and Astrophysics
• Public URL: https://nottingham-repository.worktribe.com/output/6244023
• Publisher URL: https://academic.oup.com/mnras/article-abstract/508/2/1853/6373467?redirectedFrom=fulltext
• Additional Information: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2021 The Author(s); Published by Oxford University Press on behalf of Royal Astronomical Society. All rights reserved.