Probing the Large-scale Structure with 21 cm-Galaxy Cross-bispectrum: Estimates from Simulations and Forecasts for Upcoming Cosmological Surveys

Abstract

The 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. Our analysis presents a first step toward an end-to-end analysis pipeline for the future 21 cm-galaxy cross-bispectrum observations. © 2026. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.