Predicting the Observability of Population III Stars with ELT-HARMONI via the Helium $1640{\rm\AA}$ emission line

TL;DR

This paper assesses the potential of the ELT-HARMONI spectrograph to detect Population III stars via the HeII 1640 Å emission line across various redshifts, using simulations and spectral modeling.

Contribution

It introduces a simulation-based method to predict the observability of Pop. III stars with ELT-HARMONI, considering different initial mass functions and galaxy properties.

Findings

HARMONI can detect Pop. III stars up to redshift 10 with a top-heavy IMF.

Detection is limited to galaxies with stellar masses over 10^7 M_sun for a Salpeter IMF.

Minimal flux thresholds for detection at different redshifts are established.

Abstract

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 $z=10$.