Detecting the signature of helium reionization through ³HeII 3.46 cm line-intensity mapping

¹ Institute for Theoretical Physics, Heidelberg University Philosophenweg 12 D–69120 Heidelberg, Germany
² Universität Bonn, Argelander-Institut für Astronomie Auf dem Hügel 71 53121 Bonn, Germany
³ SISSA, International School for Advanced Studies Via Bonomea 265 34136 Trieste TS, Italy
⁴ Dipartimento di Fisica – Sezione di Astronomia, Università di Trieste Via Tiepolo 11 34131 Trieste, Italy
⁵ IFPU, Institute for Fundamental Physics of the Universe Via Beirut 2 34151 Trieste, Italy
⁶ Max-Planck-Institut für Astronomie Königstuhl 17 69117 Heidelberg, Germany
⁷ Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa Chiba 277-8583, Japan
⁸ Aix Marseille Univ, CNRS, CNES, LAM Marseille, France
⁹ INAF – Astronomical Observatory of Trieste Via G.B. Tiepolo 11 I-34143 Trieste, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 21 November 2025
Accepted: 16 January 2026

Abstract

Context. Helium reionization is the most recent phase change of the intergalactic medium, yet its timing and main drivers remain uncertain. Among the probes to trace to trace how it unfolds, the 3.46 cm hyperfine line of singly ionized helium has opened the study of helium reionization to upcoming radio surveys.

Aims. We aim to evaluate the detectability of the 3.46 cm signal with radio surveys and the possible constraints it can place on helium reionization. In particular, we seek to determine whether it can distinguish between early and late helium reionization scenarios. Moreover, we performed a comprehensive study of the advantages of a single-dish versus an interferometric setup.

Methods. Using hydrodynamic simulations post-processed with radiative transfer, we constructed mock data cubes for two models of helium reionization. We computed the power spectrum of the signal and forecasted the signal-to-noise ratio for SKA-1 MID, DSA-2000, and a PUMA-like survey using each observational setup.

Results. The two scenarios produce distinct power spectra, but the faintness of the signal, largely caused by weak coupling between the spin temperature and the kinetic temperature in low-density regions of the IGM, combined with high instrumental noise, makes detection very difficult within realistic integration times for current surveys. A PUMA-like survey operating in single-dish mode could, however, detect the 3.46 cm signal with an integrated signal-to-noise ratio of a few in ≲1000 h in both scenarios.

Conclusions. Distinguishing helium reionization scenarios with 3.46 cm line-intensity mapping therefore remains challenging for current facilities. Our results, however, indicate that next-generation high-sensitivity surveys with optimized observing strategies, especially when combined with complementary probes of the intergalactic medium, could begin to place meaningful constraints on the timing and morphology of helium reionization.