Metal-polluted Population III galaxies and How to Find Them

TL;DR

This paper models the evolution of Population III galaxies to improve their detection prospects with JWST, highlighting phases with metal line emissions and proposing new diagnostics to identify candidates.

Contribution

It introduces a self-consistent galaxy formation model that predicts observable signatures of PopIII galaxies, including metal line emissions and hybrid phases, aiding their identification.

Findings

PopIII galaxies can emit metal lines during self-polluted phases.

HeII emission can last up to 20 million years in PopIII galaxies.

Nine candidate galaxies with significant metal-free stellar mass were identified in JWST data.

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 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.