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

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 $L_\alpha$, redshift uncertainty $\sigma_z$, and 21cm foreground wedge assumptions. We calculate the signal-to-noise ratio (SNR) of the 21cm-Lyman-$\alpha$ 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 $L_\alpha$, and smaller $\sigma_z$, with the FoV having the strongest impact when $\sigma_z$ is small. (ii) Under a moderate foreground wedge, photometric-like surveys yield insufficient SNR, and medium-deep ($L_\alpha\gtrsim10^{42.5}$erg s$^{-1}$), wide-area (FoV>20deg$^2$) slitless spectroscopic surveys are needed. (iii) Under an optimistic foreground wedge, detection is possible with deep ($L_\alpha\gtrsim10^{42.3}$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.