Chronicling the Reionization History at 6 ≲ z ≲ 7 with Emergent Quasar Damping Wings

Abstract

The spectra of high-redshift ( z ≳ 6) quasars contain valuable information on the progression of the Epoch of Reionization. At redshifts z < 6, the observed Lyman-series forest shows that the intergalactic medium 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 ≲ z ≲ 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 Ly α 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, x ¯ HI , while jointly fitting for the mean quasar lifetime, t Q , for each stacked spectrum. We chronicle the evolution of neutral gas fraction using the ATON (CROC) models as follows: x ¯ HI = 0.21 − 0.07 + 0.17 ( x ¯ HI = 0.10 < 10 − 4 + 0.73 ) at 〈 z 〉 = 6.10, x ¯ HI = 0.21 − 0.07 + 0.33 ( x ¯ HI = 0.57 − 0.47 + 0.26 ) at 〈 z 〉 = 6.46, and x ¯ HI = 0.37 − 0.17 + 0.17 ( x ¯ HI = 0.57 − 0.21 + 0.26 ) at 〈 z 〉 = 6.87. At the same time, we constrain the average quasar lifetime to be t Q ≲ 7 Myr across all redshift bins, in good agreement with previous studies.