The characteristic shape of damping wings during reionization

ABSTRACT Spectroscopic analysis of Ly α 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 analyse the damping wings arising from the neutral patches in the radiative transfer cosmological simulation suite cosmic reionization on computers. We find that the damping wing profile remains a tight function of volume-weighted neutral fraction xH iv, especially when xH iv 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 Δ xH iv ≲ 0.1 in the first half of reionization. Due to resonant Ly α absorption from the residual neutral hydrogen inside the ionized bubble, it may be challenging to find the starting point of the damping wing for faint sources like normal galaxies or gamma-ray bursts. However, for bright sources like quasars, it is hopeful to measure the entire damping wing and use them to measure the neutral fraction accurately.