Probing large-scale filaments with H I and 3He II

We explore the observability of the neutral hydrogen (H I) and the singly ionized isotope helium-3 (He-3 II) in the intergalactic medium (IGM) from the Epoch of Reionization down to the local Universe. The hyperfine transition of He-3 II, which is not as well known as the H I transition, has energy splitting corresponding to 8 cm. It also has a larger spontaneous decay rate than that of neutral hydrogen, whereas its primordial abundance is much smaller. Although both species are mostly ionized in the IGM, the balance between ionization and recombination in moderately high-density regions renders them abundant enough to be observed. We estimate the emission signal of both hyperfine transitions from large-scale filamentary structures and discuss the prospects for observing them with current and future radio telescopes. We conclude that H I in filaments is possibly observable even with current telescopes after 100 h of observation. On the other hand, He-3 II is only detectable with future telescopes, such as Square Kilometre Array, after the same amount of time.