Exploring HeII${\lambda}$1640 emission line properties at ${z\sim2-4}$
  Deep optical spectroscopic surveys of galaxies provide us a unique
opportunity to investigate rest-frame ultra-violet (UV) emission line
properties of galaxies at ${z \sim 2-4.5}$. Here we combine VLT/MUSE Guaranteed
Time Observations of the Hubble Deep Field South, Ultra Deep Field, COSMOS, and
several quasar fields with other publicly available data from VLT/VIMOS and
VLT/FORS2 to construct a catalogue of HeII${\lambda}$1640 emitters at
${z\sim2}$. The deepest areas of our MUSE pointings reach a ${3\sigma}$ line
flux limit of 3.1${\times}$10-19 erg/ s/ cm$^2$. After discarding broad line
active galactic nuclei we find 13 HeII${\lambda}$1640 detections from MUSE with
a median MUV = $-20.1$ and 21 tentative HeII${\lambda}$1640 detections from
other public surveys. Excluding Ly${\alpha}$, all except two galaxies in our
sample show at least one other rest-UV emission line, with
CIII]${\lambda}$1907,${\lambda}$1909 being the most prominent. We use
multi-wavelength data available in the Hubble legacy fields to derive basic
galaxy properties of our sample via spectral energy distribution fitting
techniques. Taking advantage of the high quality spectra obtained by MUSE
(${\sim10 - 30}$h of exposure time per pointing), we use photo-ionisation
models to study the rest-UV emission line diagnostics of the
HeII${\lambda}$1640 emitters. Line ratios of our sample can be reproduced by
moderately sub-solar photo-ionisation models, however, we find that including
effects of binary stars lead to degeneracies in most free parameters. Even
after considering extra ionising photons produced by extreme sub-solar
metallicity binary stellar models, photo-ionisation models are unable to
reproduce rest-frame HeII${\lambda}$1640 equivalent widths (${\sim}$ 0.2 - 10
A), thus additional mechanisms are necessary in models to match the observed
HeII${\lambda}$1640 properties.
