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 $L_α$, 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.

Figures (16)

• Figure 1: 21cm-LAE cross-power spectrum at $z=7$ and $8$ for the faint galaxy ( mhdec, left) and bright galaxy ( mhinc, right) reionisation scenarios Hutter2023b and different minimum observed Lyman-$\alpha$ luminosities of $L_\alpha^\mathrm{min}=10^{41.0}$erg/s (dotted lines), $10^{42.0}$erg/s (dash-dotted lines), $10^{42.8}$erg/s (solid lines). Shaded regions depict the observational uncertainties for a spectroscopic survey ($\sigma_z=0.001$) covering a FoV of $25$ deg$^2$ assuming the SKA-Low1 AA4 antenna layout and a moderate foreground model, see Section \ref{['sec_error']}. The cross-power spectrum values shown are interpolated from $65$ and $68$ linearly spaced $k$ bins (following the $k$ binning described in Section \ref{['sec_error']}) at redshifts $z=7$ and $8$, respectively, and smoothed with a Gaussian kernel of standard deviation $\sigma=1$ bin.
• Figure 2: Two-dimensional 21cm--galaxy cross-power uncertainties, $\sigma_\mathrm{21,gal}(k_\parallel, k_\perp)$, (see Sec. \ref{['sec_error']}). The accessible EoR window for cross-correlations depends on (1) 21cm foreground assumption and (2) galaxy survey redshift uncertainty.
• Figure 3: Signal-to-noise ratio of the 21cm--LAE cross-power spectrum $P_\mathrm{21,LAE}(k)$ as a function of wavenumber $k$ for different observational configurations. The top, middle, and bottom panels correspond to varying FoV, galaxy redshift uncertainty, $\sigma_z$, and minimum Lyman-$\alpha$, luminosity $L_\alpha^\mathrm{min}$, respectively. The 21cm signal noise is computed assuming the SKA-Low1 AA4 antenna layout and a moderate foreground model.
• Figure 4: Signal-to-noise ratio of the 21cm-LAE cross-power spectrum $P_\mathrm{21,LAE}(k)$ as a function of wavenumber $k$ for different observational configurations. The top, middle, and bottom panels correspond to varying FoV, galaxy redshift uncertainty, $\sigma_z$, and minimum Lyman-$\alpha$ luminosity, $L_\alpha^\mathrm{min}$, respectively. The 21cm signal noise is computed assuming the SKA-Low1 AA4 antenna layout and an optimistic foreground model.
• Figure 5: Total S/N of the 21cm--LAE cross-power spectrum in the mhdec reionisation scenario as a function of survey FoV and the minimum Lyman-$\alpha$ luminosity, $L_\alpha^\mathrm{min}$, shown for spectroscopic (left), grism (centre) and photometric-like (right) surveys at $z=7$. The 21cm signal noise was computed assuming the SKA-Low1 AA4 antenna layout and a moderate foreground model. Black stars mark potential LAE surveys at $z\simeq7$ that could be cross-correlated with SKA 21cm data (see also Tab. \ref{['tab_surveys']}).