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Abstract We present a new investigation of the intergalactic medium (IGM) near the end of reionization using “dark gaps” in the Ly α forest. Using spectra of 55 QSOs at z em > 5.5, including new data from the XQR-30 VLT Large Programme, we identify gaps in the Ly α forest where the transmission averaged over 1 comoving h −1 Mpc bins falls below 5%. Nine ultralong ( L > 80 h −1 Mpc) dark gaps are identified at z < 6. In addition, we quantify the fraction of QSO spectra exhibiting gaps longer than 30 h −1 Mpc, F 30 , as a function of redshift. We measure F 30 ≃ 0.9, 0.6, and 0.15 at z = 6.0, 5.8, and 5.6, respectively, with the last of these long dark gaps persisting down to z ≃5.3. Comparing our results with predictions from hydrodynamical simulations, we find that the data are consistent with models wherein reionization extends significantly below redshift six. Models wherein the IGM is essentially fully reionized that retain large-scale fluctuations in the ionizing UV background at z ≲6 are also potentially consistent with the data. Overall, our results suggest that signatures of reionization in the form of islands of neutral hydrogen and/or large-scale fluctuations in the ionizing background remain present in the IGM until at least z ≃ 5.3.
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Long Dark Gaps in the Lyβ Forest at z < 6: Evidence of Ultra-late Reionization from XQR-30 Spectra
Abstract 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.
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- PAR ID:
- 10342957
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 932
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 76
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.3847/1538-4357/ac6e60
