The properties of He IIλ1640 emitters at z ∼ 2.5-5 from the VANDELS survey

Aims. Strong He II emission is produced by low-metallicity stellar populations. Here, we aim to identify and study a sample of He IIλ 1640-emitting galaxies at redshifts of z  ∼ 2.5 − 5 in the deep VANDELS spectroscopic survey.Methods. We identified a total of 33 Bright He II emitters (S /N  >  2.5) and 17 Faint emitters (S /N W H M ) and 6 Broad (F W H M  >  1000 km s−1 ) He II emitters. We created stacks of Faint, Narrow, and Broad emitters and measured other rest-frame UV lines such as O III] and C III] in both individual galaxies and stacks. We then compared the UV line ratios with the output of stellar population-synthesis models to study the ionising properties of He II emitters.Results. We do not see a significant difference between the stellar masses, star-formation rates, and rest-frame UV magnitudes of galaxies with He II and no He II emission. The stellar population models reproduce the observed UV line ratios from metals in a consistent manner, however they under-predict the total number of He II ionising photons, confirming earlier studies and suggesting that additional mechanisms capable of producing He II are needed, such as X-ray binaries or stripped stars. The models favour subsolar metallicities (∼0.1 Z ⊙ ) and young stellar ages (106  − 107 years) for the He II emitters. However, the metallicity measured for He II emitters is comparable to that of non-He II emitters at similar redshifts. We argue that galaxies with He II emission may have undergone a recent star-formation event, or may be powered by additional sources of He II ionisation.