HeII emitters. {lambda}1640 emission

Within this context, the origin of the nebular emission remains an open question. In this work, we conduct a systematic, multi-wavelength investigation of a sample of z~2-4 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 derive 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 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 (Mup=100M_{sun}_) and stellar metallicity ~0.001 (i.e., similar to that of the gas) can reproduce the observed ionising conditions.