Probing the large-scale structure with 21 cm-galaxy cross-bispectrum: estimates from simulations and forecasts for upcoming cosmological surveys

Abstract

he redshifted 21 cm signal from the post-reionization epoch is highly non-Gaussian; thus, higher-order statistics, such as the bispectrum, are required to extract this non-Gaussian information. However, high signal-to-noise ratio (SNR) detection of the 21 cm auto-bispectrum will be hindered by the presence of residual systematics. Cross-correlating the 21 cm signal with galaxies offers a promising path to suppress this uncertainty from residual systematics and potentially increase the SNR. We present a comprehensive analysis of the H I-galaxy cross-bispectrum using the predictions of theoretical galaxy evolution models defined on large cosmological volumes. Our analysis includes the cross-bispectrum for different triangle sizes and shapes, as well as for different combinations of the H I and galaxy fields. We forecast the detectability of the 21 cm-galaxy cross-bispectrum at redshift z ≈ 1 with the Euclid-like galaxy survey and SKA-Mid observations in both the interferometric and single-dish modes of the survey. We find that the 21 cm-galaxy cross-bispectrum shows enhanced detectability compared to the 21 cm auto-bispectrum for all unique triangles in the interferometric mode of observations. We forecast a 10σ detection of the cross-bispectrum for squeezed-limit triangles and a 100σ detection for all shapes combined for scales of 0.2 Mpc−1 ≤ k1 ≤ 0.9 Mpc−1, with 100 hr of SKA-Mid observations per pointing. However, the detectability of the cross-bispectrum for large scales (k1 < 0.1 Mpc−1), which is accessible with the single-dish mode of the survey, is limited by cosmic variance. Additionally, the signal loss due to foreground removal further suppresses the detectability.