Affiliation: Institute for Theoretical Physics, Heidelberg University

Contribution: Oral

Title: Damping wings in the Lyman-α forest

Abstract: 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 $\sim 6$, 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_HI​) at the end of the EoR ($5\lesssim z\lesssim 6$) 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-$\alpha$ 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_HI​=0.19±0.07 (−0.16 +0.11​) at $1\sigma$ $(2\sigma )$ 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.

This contribution can be found here (pdf).