2025MNRAS.540.2238S

The shape of the Ly-$\alpha$ transmission in the near zone of the redshift $z=5.9896$ quasar ULAS J0148$+$0600 (hereafter J0148) is consistent with a damping wing arising from an extended neutral hydrogen island in the diffuse intergalactic medium (IGM). Here, we use simulations of late-ending reionization from Sherwood-Relics to assess the expected incidence of quasars with Ly-$\alpha$ and Ly-$\beta$ absorption similar to the observed J0148 spectrum. We find that a late end to reionization at $z=5.3$ is a necessary requirement for reproducing a Ly-$\alpha$ damping wing consistent with J0148. This occurs in ${\sim} 3$ per cent of our simulated spectra for an IGM neutral fraction $\langle x_{{\rm H} \, {\small I}}\rangle =0.14$ at $z=6$. However, using standard assumptions for the ionizing photon output of J0148, the a priori probability of drawing a simulated quasar spectrum with a Ly-$\alpha$ damping wing profile and Ly-$\alpha$ near-zone size that simultaneously matches J0148 is low, $p< 10^{-2}$. This may indicate that the ionizing emission from J0148 is variable on time-scales $t< 10^{5}\rm \, yr$, or alternatively that the Ly-$\alpha$ transmission in the J0148 near zone is impacted by the transverse proximity effect from nearby star-forming galaxies or undetected quasars. We also predict that the IGM temperature should be $T\sim 4\times 10^{4}\rm \, K$ within a few proper Mpc of the Ly-$\alpha$ near-zone edge due to recent H$\, \rm \scriptstyle I$ and He$\, \rm \scriptstyle II$ photoheating. Evidence for enhanced thermal broadening in the Ly-$\alpha$ absorption near the damping wing edge would provide further evidence that the final stages of reionization are occurring at $z< 6$.