Measuring patchy reionisation with kSZ$^2$-21 cm correlations

TL;DR

This paper proposes using the cross-correlation of the squared kSZ effect and 21 cm signals to probe the patchy nature of reionisation, demonstrating that this method can effectively detect reionisation features with upcoming observations.

Contribution

It introduces the novel approach of correlating the kSZ$^2$ field with 21 cm signals to study reionisation, accounting for varying ionisation levels and temperature effects.

Findings

Anti-correlation at low ionisation fractions due to ionised bubbles.

Positive correlation at higher ionisation levels from neutral pockets.

High signal-to-noise ratios predicted for upcoming SKA and CMB observations.

Abstract

We study cross-correlations of the kinetic Sunyaev-Zel'dovich effect (kSZ) and 21 cm signals during the epoch of reionisation (EoR) to measure the effects of patchy reionisation. Since the kSZ effect is proportional to the line-of-sight velocity, the kSZ-21 cm cross correlation suffers from cancellation at small angular scales. We thus focus on the correlation between the kSZ-squared field (kSZ$^2$) and 21 cm signals. When the global ionisation fraction is low ($x_e\lesssim 0.7$), the kSZ$^2$ fluctuation is dominated by rare ionised bubbles which leads to an anti-correlation with the 21 cm signal. When $0.8\lesssim x_e<1$, the correlation is dominated by small pockets of neutral regions, leading to a positive correlation. However, at very high redshifts when $x_e<0.15$, the spin temperature fluctuations change the sign of the correlation from negative to positive, as weakly ionised regions can have strong 21 cm signals in this case. To extract this correlation, we find that Wiener filtering is effective in removing large signals from the primary CMB anisotropy. The expected signal-to-noise ratios for a $\sim$10-hour integration of upcoming Square Kilometer Array data cross-correlated with maps from the current generation of CMB observatories with 3.4~$\mu$K arcmin noise and 1.7~arcmin beam over 100~deg$^2$ are 51, 60, and 37 for $x_e=0.2$, 0.5, and 0.9, respectively.