The effect of large optical depths on the non-Gaussian 21-cm signal from Cosmic Dawn

Abstract

During the Cosmic Dawn (CD), the H I 21-cm optical depth (τ) in the intergalactic medium can become significantly large. Consequently, the second and higher order terms of τ appearing in the Taylor expansion of the H I 21-cm differential brightness temperature (δT<inf>b</inf>) become important. This introduces additional non-Gaussianity into the signal. We study the impact of large τ on statistical quantities of H I 21-cm signal using a suite of standard numerical simulations that vary X-ray heating efficiency and the minimum halo mass required to host radiation sources. We find that the higher order terms suppress statistical quantities such as skewness, power-spectrum, and bispectrum. However, the effect is found to be particularly strong on the non-Gaussian signal. We find that the change in skewness can reach several hundred per cent in low X-ray heating scenarios, whereas for moderate and high X-ray heating models the changes are around ∼40 per cent and ∼60 per cent, respectively, for M<inf>h,min</inf> = 109 M⊙. This change is around ∼75 per cent, 25 per cent, and 20 per cent for low, moderate, and high X-ray heating models, respectively, for M<inf>h,min</inf> = 1010 M⊙. The change in bispectrum in both the halo cut-off mass scenarios ranges from ∼10 per cent to ∼300 per cent for low X-ray heating model. However, for moderate and high X-ray heating models the change remains between ∼10 per cent and ∼200 per cent for both equilateral and squeezed limit triangle configuration. Finally, we find that up to third orders of τ need to be retained to accurately model δT<inf>b</inf>, especially for capturing the non-Gaussian features in the H I 21-cm signal. © 2025 Elsevier B.V., All rights reserved.