Abstract
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 $\lambda _{\rm mfp} = 9.09^{+1.62}_{-1.28}$ 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.
Highlights
−1.28 proper Mpc and 0.75−0.45 proper Mpc (68 per cent confidence) at z = 5.1 and 6.0, respectively
Our mean free path results are most consistent with late reionization models wherein the intergalactic medium (IGM) is still 20 per cent neutral at z = 6, our measurement at z = 6.0 is even lower than these models prefer
We use a subset of the spectra from the Giant Gemini GMOS (GGG) survey presented by Worseck et al (2014)
Summary
The metagalactic UV background is a fundamental link between the intergalactic medium (IGM) and the sources of ionizing radiation (stars and active galactic nuclei). A number of observations suggest that reionization had a midpoint around z ∼7–8 and ended near z ∼ 6, or even later These include (i) the electron optical depth to CMB photons (Planck Collaboration 2020), (ii) the decline in Ly α emission from galaxies at z > 6 (e.g. Jung et al 2020; Morales et al 2021, and references therein), (iii) large-scale opacity fluctuations in the Ly α forest at z < 6 (Fan et al 2006; Becker et al.2015; Bosman et al 2018; Eilers, Davies & Hennawi 2018; Yang et al 2020), (iv) the association of large Ly α troughs at z ∼ 5.7 with. If reionization did end near or below z = 6, the mean free path at z < 6 should increase rapidly with time as large H II bubbles merge and the last remaining neutral islands are ionized (e.g. Wyithe, Bolton & Haehnelt 2008). 1990; Meiksin & Madau 1993; Haardt & Madau 1996; FaucherGiguère et al 2008; Songaila & Cowie 2010; Rudie et al 2013; Downloaded from https://academic.oup.com/mnras/article/508/2/1853/6373467 by University of Nottingham user on 07 October 2021
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