IGM damping wing constraints on the tail end of reionisation from the enlarged XQR-30 sample

Abstract

The attenuation of Ly$\hat{I}\pm$ photons by neutral hydrogen in the intergalactic medium (IGM) at $z\gtrsim 5$ continues to be a powerful probe for studying the epoch of reionisation. Given a framework to estimate the intrinsic (true) Ly$\hat{I}\pm$ emission of high-$z$ sources, one can infer the ionisation state of the IGM during reionisation. In this work, we use the enlarged XQR-30 sample of 42 high-resolution and high-SNR QSO spectra between $5.8\lesssim z\lesssim 6.6$ obtained with VLT/X-Shooter to place constraints on the IGM neutral fraction. This is achieved using our existing Bayesian QSO reconstruction framework which accounts for uncertainties such as the: (i) posterior distribution of predicted intrinsic Ly$\hat{I}\pm$ emission profiles (obtained via covariance matrix reconstruction of the Ly$\hat{I}\pm$ and N V emission lines from unattenuated high-ionisation emission line profiles; C IV, Si IV + O IV] and C III]) and (ii) distribution of ionised regions within the IGM using synthetic damping wing profiles drawn from a $1.6^3$ Gpc${}^3$ reionisation simulation. Following careful quality control, we used 23 of the 42 available QSOs to obtain constraints/limits on the IGM neutral fraction during the tail-end of reionisation. Our median and 68th percentile constraints on the IGM neutral fraction are: $0.20\begin{array}{c}+0.14\\ -0.12\end{array}$ and $0.29\begin{array}{c}+0.14\\ -0.13\end{array}$ at $z=6.15$~and 6.35. Further, we also report 68th percentile upper-limits of ${\bar{x}}_{\mathrm{H}\mathrm{I}}<0.21$, 0.20, 0.21 and 0.18 at $z=5.8,5.95,6.05$~and 6.55. These results imply reionisation is still ongoing at $5.8\lesssim z\lesssim 6.55$, consistent with previous results from XQR-30 (dark fraction and Ly$\hat{I}\pm$ forest) along with other observational probes considered in the literature.

21 pages, 9 figures and 2 tables. Accepted for publication in MNRAS