Speaker
Description
Recent observations have positioned the endpoint of the EoR at redshift $z \sim 5.3$. However, it has not been possible to discern whether this progression occurred slowly and late, with substantial neutral hydrogen (HI) clouds persisting at redshift $\sim 6$, or rapidly and earlier, driven by the fluctuating UV background, through observations of the Lyman-$\alpha$ forest.
Large-scale Gunn-Peterson (GP) absorption troughs (arising from absorption of UV radiation by HI in the intergalactic medium) are one of the most solid indicators that reionisation is not complete until redshift $z=5.3$, but whether they contain significantly neutral gas has not yet been proven.
We aim to solve this question by directly measuring, for the first time, the neutral hydrogen fraction ($x_\mathrm{H{\small{I}}}$) at the end of the EoR ($5 \lesssim z \lesssim 6$) in high-redshift quasars spectra.
For high neutral fractions ($x_\mathrm{HI}\gtrsim0.1$), 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 $z\geq7$, 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 DW signal adjacent to GP troughs at redshifts $z=5.6$ and $z=6$ through careful stacking of the Lyman-$\alpha$ forest. We present a detection and measurement of the corresponding global $x_\mathrm{HI}$, made possible by the high data quality and quantity in the XQR-30 survey.
We conclusively demonstrate the existence of substantial neutral islands near the conclusion of the EoR, unequivocally signaling a late-and-slow reionization scenario.
