On the detection of Population III galaxies: Emission Line Diagnostics for Hybrid Stellar Populations

Identifying Population III (Pop III) stars, the first generation of metal-free stars in the early Universe, remains a central challenge in astrophysics. High-ionization emission lines, such as HeII are commonly used as tracers of Pop III signatures. However, realistic galaxies may host hybrid stellar populations, including both Pop III and metal-enriched Population II (Pop II) stars, complicating the interpretation of observed spectra. We aim to investigate how hybrid Pop III/Pop II stellar populations affect emission line diagnostics and assess the detectability of Pop III stars across different galactic environments and redshifts. We select galaxies with varying Pop III-to-total mass ratios from the IllustrisTNG cosmological simulations. Using self-consistent photoionisation models, we compute integrated spectra by adopting local physical conditions from the simulations to study the resulting emission line diagnostics. We find that emission line diagnostics are strongly dependent on the relative Pop III contribution. Current diagnostics can identify galaxies dominated totally by Pop III stars but fail for systems where Pop II stars contribute significantly, introducing degeneracies in interpreting observed spectra. Our results highlight the limitations of existing emission line diagnostics in hybrid systems and emphasize the need for additional methods that account for mixed stellar populations to reliably detect Pop III stars during and after the epoch of reionization.