Choudhury, TR; Paranjape, A; Bosman, SEI; (2021) Studying the Lyman α optical depth fluctuations at z ∼ 5.5 using fast semi-numerical methods. Monthly Notices of the Royal Astronomical Society , 501 (4) pp. 5782-5796. 10.1093/mnras/stab045.

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Abstract

We present a computationally efficient and fast semi-numerical technique for simulating the Lyman α (Ly α) 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 Ly α 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 ≲ 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 sightlines 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.

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