The very first galaxies to have been formed in the Universe, which were likely to have dominated the early phases of reionisation, must harbour a population of metal-free or Pop-III stars. Although there is no direct evidence yet of Pop-III stars in galaxies at any redshift, such stars are expected to leave certain imprints on the spectra of galaxies that can be used as a starting point to identify stellar populations with near-zero metallicities. One such feature is the presence of strong and narrow He II 1640A emission in the UV spectra of galaxies. 

In this talk I will present a sample of star-forming galaxies with bright He II emission at z ~ 2-5 selected from the VANDELS survey. I will describe the properties of their rest-UV emission lines as well as their physical properties, and compare them with samples of non-He II emitters. I will then present the X-ray properties of a subset of these He II emitters, to test whether the He II emission line may be powered by weak AGN or X-ray binaries, that have long been touted as potential contributors to the production of ionising photons in star-forming galaxies. I will conclude by talking about the potential sources of He II ionising photons and what such sources could mean for the reionisation of the Universe at z > 6 in the JWST era.