The Atacama Cosmology Telescope: Detection of Patchy Screening of the Cosmic Microwave Background

Spatial variations in the cosmic electron density after reionization generatecosmic microwave background anisotropies via Thomson scattering, a processknown as the “patchy screening" effect. In this paper, we propose a newestimator for the patchy screening effect that is designed to mitigate biasesfrom the dominant foreground signals. We use it to measure thecross-correlation between unWISE galaxies and patchy screening, thelatter measured by the Atacama Cosmology Telescope and Plancksatellite. We report the first detection of the patchy screening effect, withthe statistical significance of the cross-correlation exceeding 7σ. Thismeasurement directly probes the distribution of electrons around these galaxiesand provides strong evidence that gas is more extended than the underlying darkmatter. By comparing our measurements to electron profiles extracted fromsimulations, we demonstrate the power of these observations to constrain galaxyevolution models. Requiring only the 2D positions of objects and no individualredshifts or velocity estimates, this approach is complementary to existing gasprobes, such as those based on the kinetic Sunyaev-Zeldovich effect.