Damping wing absorption associated with a giant Ly$\alpha$ trough at $z < 6$: direct evidence for late-ending reionization

TL;DR

This paper provides direct evidence of late-ending reionization at redshift below 6 by analyzing a giant Lyα trough and associated damping wing absorption in a quasar spectrum, indicating extended neutral regions in the intergalactic medium.

Contribution

It presents the first conclusive evidence linking a giant Lyα trough to damping wing absorption, supporting the existence of extended neutral islands after reionization.

Findings

Detection of damping wing absorption in a giant Lyα trough

Evidence against a compact source origin due to lack of metal lines

Implication of extended neutral regions in the IGM at z<6

Abstract

Multiple observations now suggest that the hydrogen reionization may have ended well below redshift six. While there has previously been no conclusive proof of extended neutral islands in the $z < 6$ intergalactic medium, it is possible that such islands give rise to the giant Ly$\alpha$ absorption troughs seen in the spectra of high-redshift quasars. Here we present evidence that the deepest and longest-known Ly$\alpha$ trough at $z < 6$, towards ULAS J0148+0600 (J0148), is associated with damping wing absorption. The evidence comes from a window of strong Ly$\alpha$ transmission at the edge of the J0148 proximity zone. We show that the relatively smooth profile of this transmission window is highly unlikely to arise from resonant absorption alone, but is consistent with the presence of a damping wing. We further argue that the damping wing is unlikely to arise from a compact source due to the lack of associated metal lines, and is more likely to arise from an extended neutral island associated with the giant Ly$\alpha$ trough. We investigate the physical conditions that may give rise to the strong transmission window, and speculate that it may signal an usually deep void, nearby ionizing sources, and/or the recent passage of an ionization front.