Studying the Lyman-α optical depth fluctuations at z 5.5 using fast semi-numerical methods
Abstract
We present a computationally efficient and fast semi-numerical technique for simulating the Lyman-α (Lyα) absorption optical depth in presence of neutral hydrogen "islands" left over from reionization at redshifts 5 z 6. The main inputs to the analysis are (i) a semi-numerical photon-conserving model of ionized regions during reionization (named SCRIPT) along with a prescription for simulating the shadowing by neutral islands and (ii) the fluctuating Gunn-Peterson approximation to model the Lyα absorption. Our model is then used for simulating the large-scale fluctuations in the effective optical depth as observed along sight lines towards high-z quasars. Our model is fully described by five parameters. By setting two of them to default values and varying the other three, we obtain the constraints on reionization history at 5 z 6 as allowed by the data. We confirm that reionization is not complete before z 5.6 at 2σ confidence, with the exact confidence limits depending on how the non-detections of the flux in the data are treated. We also confirm that the completion of reionization can be as late as z 5.2. With further improvements in the model and with more sight lines at z 6, we can take advantage of the computational efficiency of our analysis to obtain more stringent constraints on the ionization fraction at the tail-end of reionization.