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 (x_{HI) 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 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.}}