The properties of He II 1640 emitters at z ~ 2.5-5 from the VANDELS
  survey
  Strong He II emission is produced by low-metallicity stellar populations.
Here, we aim to identify and study a sample of He II $\lambda 1640$-emitting
galaxies at redshifts of $z \sim 2.5-5$ in the deep VANDELS spectroscopic
survey.. We identified a total of 33 Bright He II 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 \emph{Bright} emitters into 20 \emph{Narrow} (FWHM <
1000 km s$^{-1}$) and 6 \emph{Broad} (FWHM > 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.
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 \heii 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
($\sim0.1Z_\odot$) and young stellar ages ($10^6 - 10^7$ 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.
