Chronicling the reionization history with 6 < z < 7 quasars

The Epoch of Reionization (EoR) represents the last major phase transition of our universe, where the intergalactic medium (IGM) transitions from a completely neutral state following recombination to the ionized state we observe it in today. At redshifts around z < 6 the observed Lyman-series forest in quasar spectra show that the IGM is already highly ionized with volume averaged neutral gas fractions of ~10^-4, while at z > 7 the observed damping wings in the quasar spectra indicate very high neutral gas fractions of > 30%. However, the bulk of the evolution of the EoR remains largely unconstrained to date. In this work, we use a sample of 19 quasar spectra at redshifts 6 < z < 7 to close this gap in our understanding of the time evolution of the EoR. By applying a neural network based method to reconstruct the quasar emission around the partially absorbed Lyman-alpha line, we study the emergence of the Lyman-alpha damping wings with cosmic time. We further compare these predictions to two different simulation-based reionization models, which allows us to present robust constraints of the neutral gas fraction at these redshifts. Concurrently, our method also allows us to constrain the lifetimes of the quasars in our sample, which is crucial for our understanding of the formation of supermassive black holes in the early Universe.