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

Abstract

Abstract The attenuation of Lyα photons by neutral hydrogen in the intergalactic medium (IGM) at z ≳ 5 continues to be a powerful probe for studying the epoch of reionisation. Given a framework to estimate the intrinsic (true) Lyα 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 ≲ z ≲ 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 Lyα emission profiles (obtained via covariance matrix reconstruction of the Lyα 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 1.63 Gpc3 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: $0.20\substack{+0.14 -0.12}$ and $0.29\substack{+0.14 -0.13}$ at z = 6.15 and 6.35. Further, we also report 68th percentile upper-limits of $\bar{x}_{\mathrm{H\, \small {I}}{}} < 0.21$, 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 ≲ z ≲ 6.55, consistent with previous results from XQR-30 (dark fraction and Lyα forest) along with other observational probes considered in the literature.