2020A&A...636A..47S

Aims. Strong HeII emission is produced by low-metallicity stellar populations. Here, we aim to identify and study a sample of HeIIλ1640-emitting galaxies at redshifts of z∼2.5-5 in the deep VANDELS spectroscopic survey.
Methods. We identified a total of 33 Bright HeII emitters (S/N>2.5) and 17 Faint emitters (S/N<2.5) in the VANDELS survey and used the available deep multi-wavelength data to study their physical properties. After identifying seven potential AGNs in our sample and discarding them from further analysis, we divided the sample of Bright emitters into 20 Narrow (FWHM<1000km/s) and 6 Broad (FWHM>1000km/s) HeII emitters. We created stacks of Faint, Narrow, and Broad emitters and measured other rest-frame UV lines such as OIII] and CIII] 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 HeII emitters.
Results. We do not see a significant difference between the stellar masses, star-formation rates, and rest-frame UV magnitudes of galaxies with HeII and no HeII 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 HeII ionising photons, confirming earlier studies and suggesting that additional mechanisms capable of producing HeII are needed, such as X-ray binaries or stripped stars. The models favour subsolar metallicities (∼0.1Z_☉) and young stellar ages (10⁶-10⁷-years) for the HeII emitters. However, the metallicity measured for HeII emitters is comparable to that of non-HeII emitters at similar redshifts. We argue that galaxies with HeII emission may have undergone a recent star-formation event, or may be powered by additional sources of HeII ionisation.