Detecting galaxy – 21-cm cross-correlation during reionization

¹ Scuola Normale Superiore di Pisa, Piazza dei Cavallieri 7, 56126 Pisa, Italy
² Centro Nazionale “High Performance Computing, Big Data and Quantum Computing”, Via Magnanelli 40033 Casalecchio di Reno, Italy

^⋆ Corresponding author: samuel.gagnonhartman@sns.it

Received: 10 March 2025
Accepted: 13 May 2025

Abstract

The cosmic 21-cm signal promises to revolutionize studies of the Epoch of Reionization (EoR). Radio interferometers are aiming for a preliminary, low signal-to-noise (S/N) detection of the 21-cm power spectrum. Cross-correlating 21-cm with galaxies will be especially useful in these efforts, providing both a sanity check for initial 21-cm detection claims and potentially increasing the S/N due to uncorrelated residual systematics. Here we self-consistently simulate large-scale (1 Gpc³) galaxy and 21-cm fields, computing their cross-power spectra for various choices of instruments and survey properties. We use 1080 h observations with SKA-low AA* and HERA-350 as our benchmark 21-cm observations. We create mock Lyman-α narrow-band, slitless and slit spectroscopic surveys, using benchmarks from instruments such as Subaru HyperSupremeCam, Roman grism, VLT MOONS, ELT MOSAIC, and JWST NIRCam. We forecast the resulting S/N of the galaxy–21-cm cross-power spectrum, varying for each pair of instruments the galaxy survey area, depth, and the 21-cm foreground contaminated region of Fourier space. We find that the highest S/N is achievable through slitless, wide-area spectroscopic surveys, with the proposed Roman HLS survey resulting in a ∼55σ (∼13σ) detection of the cross-power with 21-cm as observed with SKA-low AA* (HERA-350), for our fiducial model and assuming ∼500 sq. deg. of overlap. Narrow-band dropout surveys are unlikely to result in a detectable cross-power, due to their poor redshift localization. Slit spectroscopy can provide a high S/N detection of the cross-power for SKA-low AA* observations. Specifically, the planned MOONRISE survey with MOONS on the VLT can result in a ∼3σ detection, while a survey of comparable observing time using MOSAIC on the ELT can result in a ∼4σ detection. Our results can be used to guide survey strategies, facilitating the detection of the galaxy–21-cm cross-power spectrum.