The Characteristic Shape of Damping Wings During Reionization

TL;DR

This study demonstrates that the damping wings of Lyα in high-redshift sources reliably indicate the universe's neutral hydrogen fraction, enabling precise reionization history measurements despite reionization inhomogeneities.

Contribution

We show that damping wing profiles are tightly correlated with the neutral fraction, allowing accurate constraints on reionization even with patchy ionization.

Findings

Damping wings correlate strongly with volume-weighted neutral fraction.

The profile remains a tight function of neutral fraction despite patchiness.

Potential to constrain neutral fraction with an accuracy of Δ<x_HI>_v ≲ 0.1.

Abstract

Spectroscopic analysis of Ly$\alpha$ damping wings of bright sources at $z>6$ is a promising way to measure the reionization history of the universe. However, the theoretical interpretation of the damping wings is challenging due to the inhomogeneous nature of the reionization process and the proximity effect of bright sources. In this Letter, we analyze the damping wings arising from the neutral patches in the radiative transfer cosmological simulation suite Cosmic Reionization on Computers (CROC). We find that the damping wing profile remains a tight function of volume-weighted neutral fraction $\left< x_{\rm HI} \right>_{\rm v}$, especially when $\left< x_{\rm HI} \right>_{\rm v}>0.5$, despite the patchy nature of reionization and the proximity effect. This small scatter indicates that with a well-measured damping wing profile, we could constrain the volume-weighted neutral fraction as precise as $\Delta \left< x_{\rm HI} \right>_{\rm v} \lesssim 0.1$ in the first half of reionization.