Work

Probing helium reionization through the 3He+ hyperfine transition line

Year: 2026

Type: article

Abstract: ABSTRACT We investigate the hyperfine transition of $^{3}\mathrm{He}^{+}$ as a promising probe of the intergalactic medium (IGM) during the final stages of helium reionization. Utilizing the most recent helium reionization simulation, we generate three-dimensional maps of the 3.5 cm (8.67 GHz) differential brightness temperature and analyse its evolution. Our ‘fiducial’ results show that the volume-averaged brightness temperature declines rapidly from ${\sim} 10^{-2}\, \mu$K (in the ‘optimistic’ approach, i.e. $T_{\rm s} = T_{\rm gas}$, ${\sim} 1\, \mu$K) at $z = 3$ to ${\sim} 8 \times 10^{-5} \,\mu$K (${\sim} 2.5 \times 10^{-3}\, \mu$K) by $z = 2.3$, tracing the He ii to He iii transition driven by quasars. The power spectrum of the 3.5 cm signal exhibits a scale-dependent evolution, peaking on small scales and declining as reionization progresses. We explore the cross-correlation of the 3.5 cm transition line with the distribution of active galactic nuclei, which shows a transition from positive to negative correlation as ionized regions grow. Although current observational upper limits lie several orders of magnitude above theoretical predictions, future radio arrays such as SKA-mid offer promising prospects. Overall, this study highlights the $^{3}\mathrm{He}^{+}$ hyperfine transition as a sensitive tracer of the thermal and ionization history of the IGM during helium reionization. The predicted signal is also subjected to additional uncertainty from spin temperature modelling, and the true amplitude is expected to lie between the limiting cases explored in this work.

Source: Monthly Notices of the Royal Astronomical Society

DOI: 10.1093/mnras/stag1709

Authors: Arghyadeep Basu, Benedetta Ciardi, Enrico Garaldi