How Massive Can a Population III Starburst Be? Simulating the First Galaxies with High Lyman-Werner Background

Observing the first generation of Population III (Pop III) stars is one of the most demanding challenges in astronomy. Indeed, Pop III stars are expected to predominantly form within faint minihalos at early times with a top-heavy initial mass function, resulting in efficient metal enrichment and a fast transition to Pop II-dominated systems. However, recent surveys with the James Webb Space Telescope (JWST) have identified galaxies at the end of the Epoch of Reionization (EoR) with possible signatures of significant Pop III star formation even at these later times. We here explore the physical conditions required to produce massive Pop III starbursts during the EoR, using cosmological radiation-hydrodynamic zoom-in simulations. We specifically focus on galaxies with a virial (dynamical) mass of $M_{\rm vir} \approx 10^{8} M_{\odot}$ at $7 \lesssim z \lesssim 8$, i.e., the atomic-cooling halos that could be potential sites for such maximal Pop III starbursts. In particular, we vary the strength of Lyman-Werner (LW) background radiation up to $J_{\rm LW} \leq 10^4J_{21}$, further imposing a high star formation efficiency (up to $\epsilon_{\rm ff} = 1.0$). Our results show that Pop III starbursts, observable in strongly-lensed survey fields like GLIMPSE, can occur in the presence of a sufficiently high LW flux (with $\gtrsim 10^3J_{21}$), leading to delayed, but intense Pop III star formation. However, even for such high LW fluxes, the Pop III starburst mass is limited to $M_{\star, \rm Pop~III} <10^6 M_{\odot}$, as strong internal metal enrichment occurs after the first Pop III supernova explosions within the simulated galaxies. While the conditions favoring observable Pop III starbursts are expected to be rare, we anticipate that future and ongoing large-volume surveys leveraging gravitational lensing, such as VENUS, will detect multiple cases of Pop III starbursts in the EoR.