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

TL;DR

This paper investigates the detectability of the He II 3.46 cm hyperfine line as a probe of helium reionization using hydrodynamical simulations with radiative transfer to build mock data cubes for Early and Late reionization histories. It computes the 3D power spectrum $P(k)$ of brightness-temperature fluctuations and forecasts prospects for SKA-1 MID, DSA-2000, and a PUMA-like survey in both single-dish and interferometric modes, finding the signal is intrinsically faint due to weak spin-temperature coupling. A PUMA-like single-dish survey could achieve a modest integrated SNR $SNR_t$ of a few within $oxed{ ext{≤}1000 ext{ h}}$, potentially distinguishing reionization histories when combined with other probes, whereas current facilities remain unlikely to detect the signal. The study highlights that next-generation, high-sensitivity surveys with optimized observing strategies, and possibly cross-correlations with other IGM tracers, are needed to place meaningful constraints on the timing and topology of helium reionization.

Abstract

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 its unfolding, the 3.46 cm hyperfine line of singly-ionized helium opens the study of helium reionization to upcoming radio surveys. 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 whether it can distinguish between early and late helium reionization scenarios. Moreover, we perform a comprehensive study of the advantages of single-dish vs. interferometric setup. Using hydrodynamical simulations post-processed with radiative transfer, we construct mock data cubes for two models of helium reionization. We compute the power spectrum of the signal and forecast the signal-to-noise ratio for SKA-1 MID, DSA-2000, and a PUMA-like survey, in both observational setups. 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. 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 IGM, could begin to place meaningful constraints on the timing and morphology of helium reionization.

Figures (8)

• Figure 1: The He I (red), He II (blue), He III (yellow) fractions for Late reionization (continuous curves) and Early reionization (dashed curves) as a function of redshift. It is noteworthy that $f_\mathrm{HeI} = 0$ consistently for the Late model, whereas in the Early model, the first and second ionization of helium occur simultaneously.
• Figure 2: Spin temperature distribution for both models at various redshifts. For each snapshot, the panels show the number of simulation voxels whose spin temperature falls within a given bin, $N$, as a function of the spin temperature. The spin temperature values corresponding to three overdensities, $\Delta_b = 1, 10, 50$, are highlighted in red, orange and blue. The bottom-right panels display the evolution of the mean spin temperature with redshift together with its standard deviation.
• Figure 3: Maps of the brightness temperature for the Late model at various redshifts, accompanied by the corresponding power spectra. The continuous curves represent the mean power spectra in the simulations, while the shaded regions show the $1\sigma$ scatter around the mean.
• Figure 4: As for Figure \ref{['fig:fig2']} (to enhance the morphology of the signal, a different color scale for the brightness temperature is used; similarly for the power spectrum measurement), assuming the Early reionization scenario.
• Figure 5: Analysis of the power spectrum of the 3.46 cm line signal at redshift z=4 for the Late reionization scenario. Shown are the measured power spectrum from the simulations (blue solid line), the spectrum after accounting for the finite pixel size (light-blue dashed line), and the spectrum after Gaussian smoothing due to the angular resolution (green dash-dotted line). The rescaled matter power spectrum is displayed both in its raw form (orange solid line) and after smoothing (orange dotted line). Shaded regions indicate the $1\sigma$ scatter across the different simulation runs. The $k$-range accessible to SKA1-MID in single-dish (interferometic) mode is highlighted in red (blue). Vertical reference scales denote the wavenumbers corresponding to the telescope's maximum angular resolution ($k_\mathrm{B}$, black-dotted line), its field of view ($k_\mathrm{FOV}$, black dash-dotted line), and its pointing area ($k_\mathrm{area}$, black dashed line). The Gaussian smoothing applied corresponds to the angular resolution of SKA1-MID at $z=4$.