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

Recent observations have positioned the endpoint of the Epoch of Reionisation (EoR) at redshift z ∼ 5.3. However, observations of the Lyman-α 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 (x_HI) at the end of the EoR (5 ≲ z ≲ 6) in high-redshift quasars spectra. For high neutral fractions x_HI≳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. 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-α forest. We use the signal to present a measurement of the corresponding global x_HI=0.19±0.07 (_-0.16^+0.11) at 1σ (2σ) at z=5.6 and a limit x_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.