Prospects for kSZ²–21 cm² Cross Correlations during Reionization

Abstract The 21 cm line and the patchy kinetic Sunyaev–Zel’dovich (kSZ) effect are promising and complementary probes of the Epoch of Reionization. A challenge for cross correlating these two signals is that foreground avoidance or removal algorithms applied to the 21 cm data inevitably sacrifice Fourier modes with long wavelengths along the line of sight (i.e., low- k ∥ modes), yet only these same modes contribute to the kSZ signal. Here we show that a suitable kSZ 2 × 21 cm 2 cross-correlation statistic nevertheless remains nonvanishing, even after filtering out the corrupted low- k ∥ Fourier modes from the 21 cm data. We simulate the kSZ 2 × 21 cm 2 cross-correlation signal across reionization-era redshifts and find distinctive redshift evolution. This signal peaks early in the reionization history, when the volume-averaged fraction is around 0.1 ≲ x HII ≲ 0.2, after which it changes sign and reaches a minimum near reionization’s midpoint ( x HII ∼ 0.5), while the signal gradually vanishes as reionization completes. These trends appear generic across three simulated models which differ in their reionization histories. We forecast the detectability of the kSZ 2 × 21 cm 2 cross-power spectrum for the HERA and SKA1-Low 21 cm experiments in combination with current and next-generation cosmic microwave background (CMB) surveys including the Simons Observatory, CMB-S4, and CMB-HD. We find that a high-significance detection (signal-to-noise ratio ≳ 5 σ ) is possible with SKA1-Low and CMB-S4.