MUSE HeII$\lambda1640$ analysis at $z=2-4$
  HeII is the most sought-after emission line to detect and characterize metal
free stellar populations. However, current stellar population/photo-ionization
models lack sufficient He$^+$ ionising photons to reproduce observed HeII
fluxes while being consistent with other emission lines. Using $\sim10-30$ hour
deep pointings from MUSE, we obtain $\sim10$ $z\sim2-4$ HeII$\lambda1640$
emitters to study their inter-stellar medium (ISM) and stellar population
properties. Emission line ratio diagnostics of our sample suggest that emission
lines are driven by star-formation in solar to moderately sub-solar ($\sim
1/20$th) metallicity conditions. However, we find that even after considering
effects from binary stars, we are unable to reproduce the HeII$\lambda1640$
equivalent widths (EWs). Our analysis suggest that extremely sub-solar
metallicities ($\sim1/200$th) are required to reproduce observed
HeII$\lambda1640$ luminosities. Thus, current stellar populations may require
alternative mechanisms such as sub-dominant active galactic nuclei (AGN) or top
heavy initial-mass-functions (IMFs) to compensate for the missing He$^+$
ionising photons.
