Chronicling the reionization history at $6\lesssim z\lesssim 7$ with emergent quasar damping wings

Abstract

The spectra of high-redshift ($z\gtrsim 6$) quasars contain valuable information on the progression of the Epoch of Reionization (EoR). At redshifts $z<6$, the observed Lyman-series forest shows that the intergalactic medium (IGM) is nearly ionized, while at $z>7$ the observed quasar damping wings indicate high neutral gas fractions. However, there remains a gap in neutral gas fraction constraints at $6\lesssim z\lesssim 7$ where the Lyman series forest becomes saturated but damping wings have yet to fully emerge. In this work, we use a sample of 18 quasar spectra at redshifts $6.0<z<7.1$ to close this gap. We apply neural networks to reconstruct the quasars' continuum emission around the partially absorbed Lyman $\alpha$ line to normalize their spectra, and stack these continuum-normalized spectra in three redshift bins. To increase the robustness of our results, we compare the stacks to a grid of models from two hydrodynamical simulations, ATON and CROC, and we measure the volume-averaged neutral gas fraction, ${\bar{x}}_{\mathrm{HI}}$, while jointly fitting for the mean quasar lifetime, $t_{\mathrm{Q}}$, for each stacked spectrum. We chronicle the evolution of neutral gas fraction using the ATON (CROC) models as follows: ${\bar{x}}_{\mathrm{HI}}=0.21_{-0.07}^{+0.17}$ (${\bar{x}}_{\mathrm{HI}}=0.10_{<10^{-4}}^{+0.73}$) at $⟨z⟩=6.10$, ${\bar{x}}_{\mathrm{HI}}=0.21_{-0.07}^{+0.33}$ (${\bar{x}}_{\mathrm{HI}}=0.57_{-0.47}^{+0.26}$) at $⟨z⟩=6.46$, and ${\bar{x}}_{\mathrm{HI}}=0.37_{-0.17}^{+0.17}$ (${\bar{x}}_{\mathrm{HI}}=0.57_{-0.21}^{+0.26}$) at $⟨z⟩=6.87$. At the same time we constrain the average quasar lifetime to be $t_{\mathrm{Q}}\lesssim 7\mathrm{Myr}$ across all redshift bins, in good agreement with previous studies.

29 pages, 14 figures

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