Probing Ultralate Reionization: Direct Measurements of the Mean Free Path over 5 < z < 6

Yongda Zhu, George D. Becker, Holly M. Christenson, Anson D’Aloisio, Sarah E.I. Bosman, Tom Bakx, Valentina D’Odorico, Manuela Bischetti, Christopher Cain, Frederick B. Davies, Rebecca L. Davies, Anna Christina Eilers, Xiaohui Fan, Prakash Gaikwad, Martin G. Haehnelt, Laura C. Keating, Girish Kulkarni, Samuel Lai, Hai Xia Ma, Andrei MesingerYuxiang Qin, Sindhu Satyavolu, Tsutomu T. Takeuchi, Hideki Umehata, Jinyi Yang

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


The mean free path of ionizing photons, λ mfp, is a critical parameter for modeling the intergalactic medium (IGM) both during and after reionization. We present direct measurements of λ mfp from QSO spectra over the redshift range 5 < z < 6, including the first measurements at z ≃ 5.3 and 5.6. Our sample includes data from the XQR-30 VLT large program, as well as new Keck/ESI observations of QSOs near z ∼ 5.5, for which we also acquire new [C ii] 158 μm redshifts with ALMA. By measuring the Lyman continuum transmission profile in stacked QSO spectra, we find λ mfp = 9.33 − 1.80 + 2.06 , 5.40 − 1.40 + 1.47 , 3.31 − 1.34 + 2.74 , and 0.81 − 0.48 + 0.73 pMpc at z = 5.08, 5.31, 5.65, and 5.93, respectively. Our results demonstrate that λ mfp increases steadily and rapidly with time over 5 < z < 6. Notably, we find that λ mfp deviates significantly from predictions based on a fully ionized and relaxed IGM as late as z = 5.3. By comparing our results to model predictions and indirect λ mfp constraints based on IGM Lyα opacity, we find that the evolution of λ mfp is consistent with scenarios wherein the IGM is still undergoing reionization and/or retains large fluctuations in the ionizing UV background well below redshift 6.

Original languageEnglish (US)
Article number115
JournalAstrophysical Journal
Issue number2
StatePublished - Oct 1 2023

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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