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arXiv · 2509.15906

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

Anne Hutter·Caroline Heneka
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Abstract

21cm-galaxy cross-correlation will play a key role in confirming the cosmological 21cm signal. We investigate which survey configurations detect the 21cm-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: the field of view (FoV), limiting luminosity of galaxy surveys $L_α$, redshift uncertainty $σ_z$, and 21cm foreground wedge assumptions. We calculate the signal-to-noise ratio (SNR) of the 21cm-Lyman-$α$ emitter (LAE) cross-power spectrum for two scenarios: one where reionisation is driven by faint or by bright galaxies. We find: (i) SNR increases with 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 insufficient SNR, and medium-deep ($L_α\gtrsim10^{42.5}$erg s$^{-1}$), wide-area (FoV>20deg$^2$) slitless spectroscopic surveys are needed. (iii) Under an optimistic foreground wedge, detection is possible with deep ($L_α\gtrsim10^{42.3}$erg s$^{-1}$), wide-area (FoV$\gtrsim80$deg$^2$) photometric-like or shallower, small-area (FoV$\simeq2-3$deg$^2$) slitless spectroscopic surveys. (iv) To distinguish the two reionisation scenarios at z=7, moderate foreground wedge scenarios require deep-wide spectroscopic surveys; under an optimistic foreground wedge, shallower, medium-area (FoV$\simeq10$deg$^2$) slitless spectroscopic surveys suffice. (v) Maximising the SNR for 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 - making surveys at z>7 more promising despite lower galaxy number densities.

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Anne Hutter, Caroline Heneka. 2026-03-01. The 21cm-galaxy cross-correlation: Realistic forecast for 21cm signal detection and reionisation constraints. https://doi.org/10.1051/0004-6361%2F202557324

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