The 21cm–galaxy cross-correlation: Realistic forecast for 21cm signal detection and reionisation constraints

¹ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, A-1180 Vienna, Austria
² Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200 Copenhagen N, Denmark
³ Cosmic Dawn Center (DAWN)
⁴ Institut für Theoretische Physik, Universität Heidelberg, Philosophenweg 16, 69120 Heidelberg, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 19 September 2025
Accepted: 27 January 2026

Abstract

Context. The 21cm–galaxy cross-correlations are expected to play a key role in the confirmation of the cosmological 21cm signal.

Aims. We investigate which survey configurations detect the 21cm–Lyman-α emitter (LAE) cross-correlation signal and assess its ability to distinguish reionisation scenarios.

Methods. Our pipeline computes observational uncertainties for the 21cm–galaxy cross-power spectrum, accounting for key survey parameters. These include field of view (FoV); limiting luminosity of galaxy surveys, L_α; redshift uncertainty, σ_z; and 21cm foreground wedge assumptions. Using this pipeline, we calculated the signal-to-noise ratio (S/N) of the 21cm–LAE cross-power spectrum for two scenarios: one where faint galaxies dominate the ionising photon budget and one where reionisation is driven by bright galaxies.

Results. We find that: (i) S/N increases with a larger FoV, fainter L_α, and smaller σ_z, with the FoV having the strongest impact when σ_z is small. (ii) Under a moderate foreground wedge, photometric-like surveys yield an insufficient S/N, and medium-deep (L_α ≳ 10^42.5 erg s⁻¹), wide-area (FoV > 20 deg²) slitless spectroscopic or spectroscopic surveys are required. (iii) Under an optimistic foreground wedge, detection is possible with deep (L_α ≳ 10^42.3 erg s⁻¹), wide-area (FoV ≳ 80 deg²) photometric-like surveys, or with shallower, small-area (FoV ≃ 2 − 3 deg²) slitless spectroscopic surveys. (iv) To distinguish the two reionisation scenarios at z = 7, moderate foreground wedge scenarios require deep-wide spectroscopic surveys; under optimistic foreground wedge, assumptions shallower (L_α ≃ 10^42.8 erg s⁻¹), medium-area (FoV ≃ 10 deg²) slitless spectroscopic surveys suffice. (v) Maximising the S/N for both detection and model discrimination requires sampling the large-scale peak of the cross-power spectrum, which shifts to larger physical scales as reionisation proceeds, and the less ionisation fronts follow the gas density, thus making surveys at z > 7, or when the Universe was more neutral (⟨χ_HII⟩> 0.5), more promising despite lower galaxy number densities.

Conclusions. Our results show that large-area spectroscopic surveys and 21cm foreground cleaning are key for using 21cm–LAE cross-correlations to constrain reionisation beyond the global ionisation state of the intergalactic medium.