LIMFAST. V. Probing Reionization with the Synergy Between 21 cm and Photometric Galaxy Surveys

Abstract

Cross-correlating the 21\,cm signal from intergalactic neutral hydrogen with the galaxy distribution is a powerful probe of the Epoch of Reionization and the physical mechanisms driving it. While upcoming surveys with the SKA-Low radio telescope and the Roman Space Telescope offer ideal synergies for this measurement, foreground mitigation of 21\,cm data requires exquisite redshift precision of galaxies, effectively excluding the rich photometrically selected samples of Lyman-break galaxies (LBGs) with typical redshift uncertainties $\sigma_z \gtrsim 0.1$. We overcome this challenge using a squared statistic of the foreground-cleaned 21\,cm signal. Squaring the filtered 21\,cm signal couples surviving high $k_\parallel$ modes into low $k_\parallel$ fluctuations accessible to photometric LBG surveys. This higher-order cross-correlation statistic remains measurable even when direct cross-correlations are strongly suppressed. Applying Fisher matrix analysis to this LBG--21\,cm$^2$ cross-correlation signal simulated by the \texttt{LIMFAST} code over $6 \lesssim z \lesssim 10$, we investigate the prospects for constraining EoR physics combining SKA-Low 21\,cm measurements and Roman photometric galaxy samples. For an overlapping survey area covering 10\,deg$^2$, we find that the LBG--21\,cm$^2$ cross-correlation is detectable at high significance across much of the EoR through 1000\,hours of SKA-Low integration with 15\,MHz bandwidth and a Roman photometric galaxy sample from the High-Latitude Wide-Area Survey (HLWAS) reaching $m_\mathrm{AB,lim}=27$ and photo-$z$ uncertainties of $\sigma_z \sim 0.1$--0.3. The characteristic sign evolution of the large-scale cross-power spectrum---driven primarily, though not exclusively, by the evolving neutral fraction---provides strong constraints on the physical parameters governing the ionization and thermal history of the IGM.