Predicting the Observability of Population III Stars with ELT-HARMONI
  via the Helium $1640{\rm\AA}$ emission line
  Population III (Pop. III) stars, as of yet, have not been detected, however
as we move into the era of extremely large telescopes this is likely to change.
One likely tracer for Pop. III stars is the HeII$\lambda1640$ emission line,
which will be detectable by the HARMONI spectrograph on the European Extremely
Large Telescope (ELT) over a broad range of redshifts ($2\leq z\leq14$). By
post-processing galaxies from the cosmological, AMR-hydrodynamical simulation
NewHorizon with theoretical spectral energy distributions (SED) for Pop. III
stars and radiative transfer (i.e. the Yggdrasil Models and CLOUDY look-up
tables respectively) we are able to compute the flux of HeII$\lambda1640$ for
individual galaxies. From mock 10 hour observations of these galaxies we show
that HARMONI will be able to detect Pop. III stars in galaxies up to $z\sim10$
provided Pop. III stars have a top heavy Initial Mass Function (IMF).
Furthermore, we find that should Pop. III stars instead have an IMF similar to
those of the Pop. I stars, the HeII$\lambda1640$ line would only be observable
for galaxies with Pop. III stellar masses in excess of $10^{7}{\,M}_\odot\,{\rm
yr}^{-1}$, average stellar age $<1{\rm Myr}$ at $z=4$. Finally, we are able to
determine the minimal intrinsic flux required for HARMONI to detect Pop. III
stars in a galaxy up to $z=10$.
