Damping wings in the Lyman-α forest: a model-independent measurement of the neutral fraction at 5.4<z<6.1

Abstract

Recent observations have positioned the endpoint of the Epoch of Reionisation (EoR) at redshift $z\sim 5.3$. However, observations of the Lyman-$\alpha$ forest have not yet been able to discern whether reionisation occurred slowly and late, with substantial neutral hydrogen persisting at redshift $\sim 6$, or rapidly and earlier, with the apparent late end driven by the fluctuating UV background. Gunn-Peterson (GP) absorption troughs are solid indicators that reionisation is not complete until $z=5.3$, but whether they contain significantly neutral gas has not yet been proven. We aim to answer this question by directly measuring, for the first time, the neutral hydrogen fraction ($x_{\mathrm{HI}}$) at the end of the EoR ($5\lesssim z\lesssim 6$) in high-redshift quasars spectra. For high neutral fractions $x_{\mathrm{HI}}\gtrsim 0.1$, GP troughs exhibit damping wing (DW) absorption extending over $1000$ km s${}^{-1}$ beyond the troughs. While conclusively detected in Lyman-$\alpha$ emission lines of quasars at $z\ge 7$, DWs are challenging to observe in the general Lyman-$\alpha$ forest due to absorption complexities and small-scale stochastic transmission features. We report the first successful identification of the stochastic DW signal adjacent to GP troughs at redshifts $z=5.6$ through careful stacking of the dark gaps in Lyman-$\alpha$ forest. We use the signal to present a measurement of the corresponding global $x_{\mathrm{HI}}=0.19\pm 0.07$ ${(}_{-0.16}^{+0.11})$ at $1\sigma$ $(2\sigma )$ at $z=5.6$ and a limit $x_{\mathrm{HI}}<0.44$ at $z=5.9$. 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.

7 pages, 4 figures, published in A&A Letters, Appendix B added to address the comments of the reviewer

References in corpus (18)

• A luminous quasar at a redshift of z = 7.085
• Model-independent evidence in favor of an end to reionization by z~6
• Evidence of patchy hydrogen reionization from an extreme Ly$\alpha$ trough below redshift six
• Pōniuā'ena: A Luminous $z=7.5$ Quasar Hosting a 1.5 Billion Solar Mass Black Hole
• Hydrogen reionisation ends by $z=5.3$: Lyman-$\alpha$ optical depth measured by the XQR-30 sample
• Inferences on the Timeline of Reionization at z~8 From the KMOS Lens-Amplified Spectroscopic Survey
• A Significantly Neutral Intergalactic Medium Around the Luminous z=7 Quasar J0252-0503
• Measurements of the z ~ 6 Intergalactic Medium Optical Depth and Transmission Spikes Using a New z > 6.3 Quasar Sample
• Long Dark Gaps in the Ly$\beta$ Forest at $z<6$: Evidence of Ultra Late Reionization from XQR-30 Spectra
• IGM damping wing constraints on reionisation from covariance reconstruction of two $z\gtrsim 7$ QSOs
• (Nearly) Model-Independent Constraints on the Neutral Hydrogen Fraction in the Intergalactic Medium at $z\sim 5-7$ Using Dark Pixel Fractions in Ly$\alpha$ and Ly$\beta$ Forests
• XQR-30: the ultimate XSHOOTER quasar sample at the reionization epoch
• Chasing the Tail of Cosmic Reionization with Dark Gap Statistics in the Ly$\alpha$ Forest over $5<z<6$
• A comparison of quasar emission reconstruction techniques for $z\ge 5.0$ Lyman-$\alpha$ and Lyman-$\beta$ transmission
• Damping Wing-Like Features in the Stacked Ly$\alpha$ Forest: Potential Neutral Hydrogen Islands at $z<6$
• The XQR-30 Metal Absorber Catalog: 778 Absorption Systems Spanning 2 < z < 6.5
• How to Search for Islands of Neutral Hydrogen in the $z\sim 5.5$ IGM
• CoReCon: an open, community-powered collection of Reionization constraints

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