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Recent observations have positioned the endpoint of the Epoch of Reionisation (EoR) at redshift z 5. 3. However, observations of the Lyman- forest have not yet been able to discern whether reionisation occurred slowly and late, with substantial neutral hydrogen persisting at redshift 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 (xHI) at the end of the EoR (5 z 6) in high-redshift quasars spectra. For high neutral fractions xHI0. 1, GP troughs exhibit damping wing (DW) absorption extending over 1000 km s^-1 beyond the troughs. While conclusively detected in Lyman- emission lines of quasars at z7, DWs are challenging to observe in the general Lyman- 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- forest. We use the signal to present a measurement of the corresponding global xHI=0. 190. 07 (-₀. ₁₆^+0. 11) at 1 (2) at z=5. 6 and a limit xHI<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.
Spina et al. (Mon,) studied this question.
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