Metal-polluted Population III Galaxies and How to Find Them

Abstract 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 He II 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 [O III ]/H β ≈ 1, which opens the pool of candidates to more easily detectable sources. We predict that the high He II emission of PopIII galaxies can last up to ≈20 Myr and that it is partly maintained in the “hybrid” phase, when PopIII and Population II stars coexist in the host galaxy. We propose novel diagnostics involving ultraviolet metal lines to select PopIII candidates in high- z JWST surveys. In JADES, we identify nine 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.