2024ApJ...960...10T

We present the first constraints on the cross-correlation power spectrum of the He II (⁺He³) signal strength using the redshifted 8.67 GHz hyperfine transition between z = 2.9 and 4.1 and with interferometric data obtained from the public archive of the Australia Telescope Compact Array (ATCA). 210 hr of observations of the primary calibrator source B1934-638 were extracted from data obtained with the telescope from 2014-2021, and coherently combined in a power spectrum pipeline to measure the He II power across a range of spatial scales, and at three redshifts that span the period of helium reionization. Our best limit places the fluctuation of the brightness temperature to be less than 557 μK on spatial scales of 30' at z = 2.91, and less than 755 μK on scales of 30' at z = 4.14 (2σ noise limited). We measure a temperature of 489 μK at z = 2.91. ATCA's few antennas and persistent remaining radio frequency interference in the data prevent deeper integrations improving the results. This work is a proof of principle to demonstrate how this type of experiment can be undertaken to reach the 0.01-1 μK level expected for the helium signal at z ∼ 4.