Probing Helium Reionization Through the 3He+ Hyperfine Transition Line

We investigate the hyperfine transition of 3He+ as a promising probe of the IGM during the final stages of helium reionization. Utilising the most recent helium reionization simulation, we generate three-dimensional maps of the 3.5cm (8.67 GHz) differential brightness temperature and analyze its evolution. Our ‘fiducial’ results show that the volume-averaged brightness temperature declines rapidly from ~10−2μK (in the ‘optimistic’ approach, i.e. Ts = Tgas, ~1μK) at z = 3 to ~8 × 10−5μK (~2.5 × 10−3μK) by z = 2.3, tracing the He ii to He iii transition driven by quasars. The power spectrum of the 3.5cm signal exhibits a scale-dependent evolution, peaking on small scales and declining as reionization progresses. We explore the cross-correlation of the 3.5cm transition line with the distribution of AGNs, 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 3He+ hyperfine transition as a sensitive tracer of the thermal and ionization history of the IGM during helium reionization. The predicted signal is also subject to additional uncertainty from spin temperature modelling, and the true amplitude is expected to lie between the limiting cases explored in this work.