A needle in a haystack? Catching Pop III stars in the Epoch of Reionization. I. Pop III star forming environments

Despite extensive search efforts, direct observations of the first (Pop\ III) stars have not yet succeeded. Theoretical studies have suggested that late Pop\ III star formation is still possible in pristine clouds of high-mass galaxies, coexisting with Pop\ II stars, down to the Epoch of Reionization (EoR). Here we reassess this finding by exploring Pop\ III star formation in six $50h^{-1} ~ \si{cMpc}$ simulations performed with the hydrodynamical code \texttt{dustyGadget}. We find that Pop\ III star formation ($\sim 10^{-3.4} - 10^{-3.2} ~ \si{M_\odot.yr^{-1}.cMpc^{-3}}$) is still occurring down to $z \sim 6 - 8$, i.e. well within the reach of deep JWST surveys. At these epochs, $\gtrsim 10 \%$ of the rare massive galaxies with $M_\star \gtrsim 3 \times 10^9 ~ \si{M_\odot}$ are found to host Pop\ III stars, although with a Pop\ III/Pop\ II mass fraction $\lesssim 0.1 \%$. Regardless of their mass, Pop\ III hosting galaxies are mainly found on the main sequence, at high star formation rates, probably induced by accretion of pristine gas. This scenario is also supported by their increasing star formation histories and their preferential location in high-density regions of the cosmic web. Pop\ III stars are found both in the outskirts of metal-enriched regions and in isolated, pristine clouds. In the latter case, their signal may be less contaminated by Pop\ IIs, although its detectability will strongly depend on the specific line-of-sight to the source, due to the complex morphology of the host galaxy and its highly inhomogeneous dust distribution.