Probing large scale filaments with HI and $^3$HeII

TL;DR

This paper investigates the potential to observe large-scale filaments in the intergalactic medium through the hyperfine transitions of neutral hydrogen and helium-3, assessing their detectability with current and future radio telescopes.

Contribution

It provides estimates of the emission signals from HI and $^3$HeII in cosmic filaments and evaluates their observability with existing and upcoming radio telescope facilities.

Findings

HI in filaments could be observed with current telescopes after 100 hours.

$^3$HeII detection requires future telescopes like SKA.

Both species are abundant enough in certain regions for potential detection.

Abstract

We explore the observability of the neutral hydrogen (HI) and the singly-ionized isotope helium-3 ($^3$HeII) in the intergalactic medium (IGM) from the Epoch of Reionization down to the local Universe. The hyperfine transition of $^3$HeII, which is not as well known as the HI 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 HI in filaments is possibly observable even with current telescopes after 100 hours of observation. On the other hand, $^3$HeII is only detectable with future telescopes, such as SKA, after the same amount of time.