Damping Wings in the Lyman-α Forest: a Model-Independent Measurement of the Neutral Fraction at 5.4<Z<6.1

Context. Recent observations have positioned the end point of the Epoch of Reionization (EoR) at a redshift of z similar to 5.3. However, observations of the Lyman-alpha forest have not yet been able to discern whether reionization occurred slowly and late, with substantial neutral hydrogen persisting at a redshift of similar to 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 less than or similar to z less than or similar to 6) in high-redshift quasar spectra. Methods. For high neutral fractions, x(HI) greater than or similar to 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 >= 7, DWs are challenging to observe in the general Lyman-alpha 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-alpha forest (S/N = 6.3). We use the signal to present a measurement of the corresponding global x(HI )= 0.19 +/- 0.07 ((+0.11)(-0.16)) at 1 sigma (2 sigma) at z = 5.6 and a limit of x(HI) < 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.