We present the first constraints on the cross-correlation power spectrum of HeII (+He3) signal strength using the redshifted 8.67GHz hyperfine transition between z=2.9 and z=4.1 and with interferometric data obtained from the public archive of the Australia Telescope Compact Array. 210 hours 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 HeII power across a range of spatial scales, and at three redshifts that span the period of Helium reionization. Our best limit places the brightness temperature fluctuation to be less than 557μK on spatial scales of 30 arcmin at z=2.91, and less than 755μK on scales of 30 arcmin at z=4.14 (2-sigma noise-limited). We measure a temperature of 489μK at z=2.91. ATCA's few antennas and persistent remaining RFI 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--1μK level expected for the Helium signal at z∼4.
@article{arxiv.2309.05960,
title = {First constraints on Helium $^{+}{\rm He}^3$ evolution in $z=3-4$ using the 8.67GHz hyperfine transition},
author = {Cathryn M. Trott and Randall B. Wayth},
journal= {arXiv preprint arXiv:2309.05960},
year = {2023}
}