Metal-polluted Population III galaxies and How to Find Them

Observing Population III (hereafter PopIII) galaxies, the hosts of first-generation stars, remains challenging even with the JWST. The current few candidates have been identified through the combination of a prominent HeII emission and the absence of metal lines, a well-known but extremely brief signature of metal-free systems. Here, we accurately model the evolution of the emission from PopIII galaxies to increase the number of candidates in JWST observations. To achieve this, we employ a locally calibrated galaxy-formation model that self-consistently follows the star formation and chemical evolution initiated by the first stars. We find that PopIII galaxies can emit metal lines in their ``self-polluted'' phase, while galaxies host only metal-free stars but the gas has been chemically-enriched by the first supernovae. In this phase, PopIII galaxies have $\rm [OIII]/H\beta \approx 1$, which opens the pool of candidates to more easily detectable sources. We predict that the high HeII emission of PopIII galaxies can last up to $\rm \approx 20 \, Myrs$ and that it is partly maintained in the ``hybrid'' phase, when PopIII and PopII stars co-exist in the host galaxy. We propose novel diagnostics involving UV metal lines to select PopIII candidates in high-z JWST surveys. In JADES, we identify 9 candidate galaxies with $>25\%$ of their stellar mass in metal-free stars, showcasing the effectiveness of our method. Ultimately, the key to discovering PopIII galaxies could be to catch them during their first episodes of chemical enrichment.