Legacy of star formation in the pre-reionization universe

TL;DR

This study uses advanced simulations to explore early star formation, revealing the behavior of Population III and II stars, their contribution to cosmic star formation rates, and implications for future observations with JWST.

Contribution

Introduces new sub-grid models for Pop III and II star formation in simulations, providing insights into their roles and observability in the pre-reionization universe.

Findings

Pop II SFRD matches observed values within a factor of 2 at z~7-10.

Pop III SFRD plateaus at ~10^{-3} Msun/yr/Mpc^3 by z~10.

Pop III-only galaxies likely exist beyond JWST detection limits.

Abstract

We utilize GIZMO, coupled with newly developed sub-grid models for Population~III (Pop~III) and Population~II (Pop~II), to study the legacy of star formation in the pre-reionization Universe. We find that the Pop~II star formation rate density (SFRD), produced in our simulation ($\sim 10^{-2}\ \Msun{\rm yr^{-1}\, Mpc^{-3}}$ at $z\simeq 10$), matches the total SFRD inferred from observations within a factor of $<2$ at $7\lesssim z \lesssim10$. The Pop~III SFRD, however, reaches a plateau at $\sim10^{-3}\ \Msun{\rm yr^{-1}\, Mpc^{-3}}$ by $z\approx10$, remaining largely unaffected by the presence of Pop~II feedback. At $z$=7.5, $\sim 20\%$ of Pop~III star formation occurs in isolated haloes which have never experienced any Pop~II star formation (i.e. primordial haloes). We predict that Pop~III-only galaxies exist at magnitudes $M_{\rm UV}\gtrsim -11$, beyond the limits for direct detection with the {\it James Webb Space Telescope (JWST)}. We assess that our stellar mass function (SMF) and UV luminosity function (UVLF) agree well with the observed low mass/faint-end behavior at $z=8$ and $10$. However, beyond the current limiting magnitudes, we find that both our SMF and UVLF demonstrate a deviation/turnover from the expected power-law slope ($M_{\rm UV,turn}= -13.4\pm1.1$ at $z$=10). This could impact observational estimates of the true SFRD by a factor of $2 (10)$ when integrating to $M_{\rm UV} = -$12 ($-$8) at $z\sim 10$, depending on integration limits. Our turnover correlates well with the transition from dark matter haloes dominated by molecular cooling to those dominated by atomic cooling, for a mass $M_{\rm halo}\approx 10^{8} \Msun$ at $z\simeq 10$.