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Description
During the Epoch of Reionisation (EoR), the ultraviolet radiation from the first stars and galaxies ionised the neutral hydrogen of the intergalactic medium, which itself can emit radiation through the 21 cm hyperfine transition. Due to this, the 21 cm signal is a direct probe of the first stars in the early Universe and a key science goal for the future Square Kilometre Array (SKA). However, observing and interpreting this signal is a notoriously difficult task.
Another high-potential probe is the patchy kinetic Sunyaev-Zel'dovich effect (pkSZ). Induced by the scattering of Cosmic Microwave Background (CMB) photons with a medium of free electrons produced during the EoR, the effect altered the small-scale CMB temperature anisotropies, imprinting information on the growth of ionising bubbles during the EoR. While measurements of the pkSZ angular power spectrum by Reichardt et al. (2021) have reported a $3\sigma$ constraint of $D^{pkSZ}_{l=3000}= 3.0\pm1.0 \mu$K$^2$, the results are also subject to modelling uncertainties.
We propose an effective parametric model that establishes a connection between the 21 cm and pkSZ power spectra. Using this model to jointly fit mock 21 cm and pkSZ data points, we confirm that these two observables exhibit complementary characteristics, leading to significantly improved constraints on reionisation compared to analysing each data set separately. Our findings demonstrate that a few well-informed low-redshift (eg., z<8) measurements of the 21 cm power spectrum at $k\approx0.1$ cMpc$^{-1}$ and the pkSZ power spectrum can precisely determine the reionisation history of the Universe.
