Damping wings in the Lyman-α forest: A model-independent measurement of the neutral fraction at 5.4 < z < 6.1
Abstract
Context. Recent observations have positioned the end point of the Epoch of Reionization (EoR) at a redshift of z ∼ 5.3. However, observations of the Lyman-α forest have not yet been able to discern whether reionization occurred slowly and late, with substantial neutral hydrogen persisting at a redshift of ∼6, or rapidly and earlier, with the apparent late end driven by the fluctuating ultraviolet background. Gunn-Peterson (GP) absorption troughs are solid indicators that reionization is not complete until z = 5.3, but whether they contain significantly neutral gas has not yet been proven.
Aims: We aim to answer this question by directly measuring, for the first time, the neutral hydrogen fraction (xHI) at the end of the EoR (5 ≲ z ≲ 6) in high-redshift quasar spectra.
Methods: For high neutral fractions, xHI ≳ 0.1, GP troughs exhibit damping wing (DW) absorption extending over 1000 km s−1 beyond the troughs. While conclusively detected in Lyman-α emission lines of quasars at z ≥ 7, DWs are challenging to observe in the general Lyman-α forest due to absorption complexities and small-scale stochastic transmission features.
Results: We report the first successful identification of the stochastic DW signal adjacent to GP troughs at redshifts of z = 5.6 through careful stacking of the dark gaps in the Lyman-α forest (S/N = 6.3). We use the signal to present a measurement of the corresponding global xHI = 0.19 ± 0.07 (−0.16+0.11) at 1σ (2σ) at z = 5.6 and a limit of xHI < 0.44 at z = 5.9.
Conclusions: The detection of this signal demonstrates the existence of substantially neutral islands near the conclusion of the EoR, unequivocally signaling a late-and-slow reionization scenario.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- August 2024
- DOI:
- 10.1051/0004-6361/202450798
- arXiv:
- arXiv:2405.12273
- Bibcode:
- 2024A&A...688L..26S
- Keywords:
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- intergalactic medium;
- quasars: absorption lines;
- dark ages;
- reionization;
- first stars;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- 7 pages, 4 figures, published in A&