Damping Wing-Like Features in the Stacked Ly$\alpha$ Forest: Potential Neutral Hydrogen Islands at $z<6$

TL;DR

This study finds evidence of neutral hydrogen islands at redshift around 5.8 by analyzing damping wing-like features in the stacked Lyα forest, suggesting reionization was not fully complete below redshift 6.

Contribution

The paper introduces a novel method of stacking Lyα forests at the edges of dark gaps to detect neutral hydrogen islands, providing new evidence for late reionization.

Findings

Detection of damping wing-like absorption profiles at z=5.8.

Neutral hydrogen fraction estimated to be at least 6.1%.

Reionization likely extends below redshift 6.

Abstract

Recent quasar absorption line observations suggest that reionization may end as late as $z \approx 5.3$. As a means to search for large neutral hydrogen islands at $z<6$, we revisit long dark gaps in the Ly$\beta$ forest in VLT/X-Shooter and Keck/ESI quasar spectra. We stack the Ly$\alpha$ forest corresponding to both edges of these Ly$\beta$ dark gaps and identify a damping wing-like extended absorption profile. The average redshift of the stacked forest is $z=5.8$. By comparing these observations with reionization simulations, we infer that such a damping wing-like feature can be naturally explained if these gaps are at least partially created by neutral islands. Conversely, simulated dark gaps lacking neutral hydrogen struggle to replicate the observed damping wing features. Furthermore, this damping wing-like profile implies that the volume-averaged neutral hydrogen fraction must be $\langle x_{\rm HI} \rangle \geq 6.1 \pm 3.9\%$ at $z = 5.8$. Our results offer robust evidence that reionization extends below $z=6$.