2025A&A...702A...3G

The study of high-redshift galaxies provides critical insights into the early stages of cosmic evolution, particularly during what is known as cosmic noon, when star formation activity reached its peak. Within this context, the origin of the nebular He II emission remains an open question. For this work, we conducted a systematic multi-wavelength investigation of a sample of z ∼ 2-4 He II λ1640 Å emitters from the MUSE Hubble Ultra Deep Field surveys, utilising both MUSE and JWST/NIRSpec data and extending the sample presented by previous studies. We derived gas-phase metallicities and key physical properties, including electron densities, temperatures, and the production rates of hydrogen- and He⁺-ionising photons. Our results suggest that a combination of factors, such as stellar mass, initial mass function, stellar metallicity, and stellar multiplicity, likely contributes to the origin of the observed nebular He II emission. Specifically, for our galaxies with higher gas-phase metallicity (12 + log(O/H) ≳ 7.55), we find that models for binary population with Salpeter IMF (M_up = 100 M_⊙) and stellar metallicity Z_⋆ ≈ 10^–3 (i.e. similar to that of the gas) can reproduce the observed He II ionising conditions. However, at lower metallicities, models for binary populations with a 'top-heavy' initial mass function (M_up = 300 M_⊙) and Z_⋆ much lower than that of the gas (10^–4 < Z_⋆ < 10^–5) are required to fully account for the observed He II ionising photon production. These results reinforce that the He II ionisation keeps challenging current stellar populations, and the He II ionisation problem persists in the very low-metallicity regime.