Catching the Nebular Needle in a Polluted Haystack: Line-emission Signatures from Population III-forming Pockets around Massive Galaxies at the End of Reionization

TL;DR

This study explores how to detect Population III stars around massive galaxies at high redshift using line emission signatures, highlighting the potential of JWST observations to identify these elusive first-generation stars.

Contribution

It demonstrates that strong HeII1640 emission can indicate Pop III clusters in high-redshift galaxies, and discusses strategies for their detection amidst Pop II star contamination.

Findings

Pop III clusters produce detectable HeII1640 emission with JWST.

Metal lines alone cannot confirm Pop III presence due to Pop II contamination.

Spatially resolved observations can identify pristine pockets of Pop III stars.

Abstract

Finding the first generation of (Population III or Pop III) stars is one of the most ambitious and exciting challenges of astrophysics. JWST opened concrete prospects for their detection during the Epoch of Reionization (EoR), where increasing evidence suggests that residual Pop III formation may persist, even within pristine pockets of high-mass halos, due to inhomogeneous enrichment. However, the identification of Pop III stars within globally enriched environments will be challenging. We investigate the detectability of a subdominant Pop III component in/around massive ($M_\star \gtrsim 10^9 ~\mathrm{M_\odot}$) galaxies at $z \approx 6.5 - 9$ from the dustyGadget cosmological simulation suite, and the confusion arising from second-generation (Pop II) stars in their surroundings. We find that young ($\lesssim 1$ Myr), massive ($M_\mathrm{III} \sim 6 \times 10^5 ~\mathrm{M_\odot}$) Pop III clusters forming within these galaxy environments are responsible for strong HeII1640 line emission ($L_\mathrm{HeII1640} \gtrsim 10^{41} ~\mathrm{erg \, s^{-1}}$), which would be detectable with $\approx 10 (50)$ h of medium-resolution observations with NIRSpec/IFU at $z \approx 6 (10)$. These bright luminosities cannot be produced by standard Pop II populations alone. On the other hand, the dominant Pop II component within massive ``hybrid'' Pop III hosts powers strong metal line emission ($L_\mathrm{[OIII]5007} \gtrsim 10^{42} ~\mathrm{erg \, s^{-1}}$), indicating that the detection of metal lines alone cannot exclude the presence of Pop IIIs in high-$z$ galaxy environments. We further discuss candidate selection strategies based on Ly$\alpha$, H$\alpha$ and H$\beta$ emission, and how spatially resolved observations may enable the detection of isolated, pristine pockets in the outskirts of massive halos.