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

The 21cm--galaxy cross-correlations are expected to play a key role in the confirmation of the cosmological 21cm signal. We investigate which survey configurations detect the 21cm--Lyman-α emitter (LAE) cross-correlation signal and assess its ability to distinguish reionisation scenarios. 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_ 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. We find that: (i) S/N increases with a larger FoV, fainter L_α, and smaller σ_ ≃ 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 (łanglȩhi_ ̊angle > 0. 5), more promising despite lower galaxy number densities. with the FoV having the strongest impact when σ_ >20 deg²) slitless spectroscopic or spectroscopic surveys are required. (iii) Under an optimistic foreground wedge, detection is possible with deep (L_α), wide-area (mathrm is small. (ii) Under a moderate foreground wedge, photometric-like surveys yield an insufficient S/N, and medium-deep (L_α), wide-area (mathrm s -1 FoV s -1 FoV deg²) photometric-like surveys, or with shallower, small-area (mathrm FoV 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), medium-area (mathrm s -1 FoV HII 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.