Studying the Lyman α optical depth fluctuations at z ∼ 5.5 using fast semi-numerical methods

We present a computationally efficient and fast semi-numerical technique for simulating the Lyman alpha (Ly alpha) absorption optical depth in presence of neutral hydrogen 'islands' left over from reionization at redshifts 5 less than or similar to z less than or similar to 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 Ly alpha absorption. Our model is then used for simulating the large-scale fluctuations in the effective optical depth as observed along sightlines 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 less than or similar to z less than or similar to 6 as allowed by the data. We confirm that reionization is not complete before z similar to 5.6 at greater than or similar to 2 sigma 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 similar to 5.2. With further improvements in the model and with more sightlines at z similar to 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.

Automated summary

A computationally efficient method is proposed in this study to simulate Lyman alpha absorption optical depth in the presence of neutral hydrogen "islands" leftover from reionization at redshifts between 5 and 6. By analyzing large-scale fluctuations in optical depth, it is confirmed at greater than or similar to 2 sigma confidence that reionization is not complete before around 5.6, with a possibility of completion as late as around 5.2.