Stacking 21 cm Maps around Ly <i>α</i> Emitters during Reionization: Prospects for a Cross-correlation Detection with the Hydrogen Epoch of Reionization Array

Abstract Observations of the redshifted 21 cm line during the Epoch of Reionization will open a new window to probe the intergalactic medium during the formation of the first stars, galaxies, and black holes. A particularly promising route to an initial detection is to cross-correlate tomographic 21 cm maps with spectroscopically confirmed Ly α emitters (LAEs). High-redshift LAEs preferentially reside in ionized bubbles that are strongly anticorrelated with the surrounding neutral regions traced by 21 cm observations. In this work, we study the prospect of detecting such a cross-correlation signal by stacking 21 cm image cubes around LAEs using a current-generation 21 cm instrument—the Hydrogen Epoch of Reionization Array. Our forecast adopts a realistic mapping pipeline to generate foreground-free 21 cm image cubes. The statistical properties of these images, arising from the complex instrumental response, are carefully accounted for. We further introduce a physically motivated signal template calibrated on the thesan radiation-hydrodynamic simulations, which connects the cross-correlation amplitude to the global neutral fraction. Our results show that a sample of ∼50 spectroscopically confirmed LAEs is sufficient to begin constraining the reionization history. These results represent an important preparatory step toward joint analyses of 21 cm experiments with upcoming wide-area, high-redshift galaxy surveys from Euclid and the Nancy Grace Roman Space Telescope.