Long Dark Gaps in the Lyβ Forest at z < 6: Evidence of Ultra-late Reionization from XQR-30 Spectra

Long Dark Gaps in the Lyβ Forest at z < 6: Evidence of Ultra-late Reionization from XQR-30 Spectra

We present a new investigation of the intergalactic medium near reionization using dark gaps in the Ly β forest. With its lower optical depth, Ly β offers a potentially more sensitive probe to any remaining neutral gas compared to the commonly used Ly α line. We identify dark gaps in the Ly β forest...

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Main Authors: Yongda Zhu, George D. Becker, Sarah E. I. Bosman, Laura C. Keating, Valentina D’Odorico, Rebecca L. Davies, Holly M. Christenson, Eduardo Bañados, Fuyan Bian, Manuela Bischetti, Huanqing Chen, Frederick B. Davies, Anna-Christina Eilers, Xiaohui Fan, Prakash Gaikwad, Bradley Greig, Martin G. Haehnelt, Girish Kulkarni, Samuel Lai, Andrea Pallottini, Yuxiang Qin, Emma V. Ryan-Weber, Fabian Walter, Feige Wang, Jinyi Yang
Format: Article
Language:English
Published: IOP Publishing 2022-01-01
Series:The Astrophysical Journal
Subjects:
Reionization
Intergalactic medium
Quasar absorption line spectroscopy
High-redshift galaxies
Online Access:https://doi.org/10.3847/1538-4357/ac6e60
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Summary:We present a new investigation of the intergalactic medium near reionization using dark gaps in the Ly β forest. With its lower optical depth, Ly β offers a potentially more sensitive probe to any remaining neutral gas compared to the commonly used Ly α line. We identify dark gaps in the Ly β forest using spectra of 42 QSOs at z _em > 5.5, including new data from the XQR-30 VLT Large Programme. Approximately 40% of these QSO spectra exhibit dark gaps longer than 10 h ^−1 Mpc at z ≃ 5.8. By comparing the results to predictions from simulations, we find that the data are broadly consistent both with models where fluctuations in the Ly α forest are caused solely by ionizing ultraviolet background fluctuations and with models that include large neutral hydrogen patches at z < 6 due to a late end to reionization. Of particular interest is a very long ( L = 28 h ^−1 Mpc) and dark ( τ _eff ≳ 6) gap persisting down to z ≃ 5.5 in the Ly β forest of the z = 5.85 QSO PSO J025−11. This gap may support late reionization models with a volume-weighted average neutral hydrogen fraction of 〈 x _H I 〉 ≳ 5% by z = 5.6. Finally, we infer constraints on 〈 x _H I 〉 over 5.5 ≲ z ≲ 6.0 based on the observed Ly β dark gap length distribution and a conservative relationship between gap length and neutral fraction derived from simulations. We find 〈 x _H I 〉 ≤ 0.05, 0.17, and 0.29 at z ≃ 5.55, 5.75, and 5.95, respectively. These constraints are consistent with models where reionization ends significantly later than z = 6.
ISSN:1538-4357

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