Quantifying and mitigating the effect of snapshot interval in light-cone Epoch of Reionization 21-cm simulations

Abstract

The Epoch of Reionization (EoR) neutral Hydrogen (HI) 21-cm signal evolves significantly along the line-of-sight (LoS) due to the light-cone (LC) effect. It is important to accurately incorporate this in simulations in order to correctly interpret the signal. The 21-cm LC simulations are typically produced by stitching together slices from a finite number $(N_{\mathrm{RS}})$ of "reionization snapshot'', each corresponding to a different stage of reionization. In this paper, we have quantified the errors in the 21-cm LC simulation due to the finite value of $N_{\mathrm{RS}}$. We show that this can introduce large discontinuities $(>200\%)$ at the stitching boundaries when $N_{\mathrm{RS}}$ is small $(=2,4)$ and the mean neutral fraction jumps by $\delta {\bar{x}}_{\mathrm{HI}}=0.2,0.1$ respectively at the stitching boundaries. This drops to $17\%$ for $N_{\mathrm{RS}}=13$ where $\delta {\bar{x}}_{\mathrm{HI}}=0.02$. We find that we can achieve $\delta {\bar{x}}_{\mathrm{HI}}\le 0.01$ with $N_{\mathrm{RS}}=26$, and we use this as the reference for comparing the other simulations. We present and also validate a method for mitigating this error by increasing $N_{\mathrm{RS}}$ without a proportional increase in the computational costs which are mainly incurred in generating the dark matter and halo density fields. Our method generates these fields only at a few redshifts, and interpolates them to generate reionization snapshots at closely spaced redshifts. We use this to generate 21-cm LC simulations with $N_{\mathrm{RS}}=51,101$ and $201$, and show that the errors go down as $N_{\mathrm{RS}}^{-1}$.

16 pages, 9 figures, accepted for publication in JoAA

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