How probable is the Lyman-$\hat{I}\pm$ damping wing in the spectrum of the redshift z = 5.9896 quasar ULAS J0148+0600?

Abstract

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

16 pages, 9 figures. Accepted for publication in MNRAS