Legacy of star formation in the pre-reionization universe

We utilize the hydrodynamic and N-body code GIZMO, coupled with newly developed sub-grid Legacy models for Population~III (Pop~III) and Population~II (Pop~II), specifically designed for meso-scale cosmological volume simulations, 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 ($\sim 10^{-2}\ M_\odot{\rm yr^{-1}\, Mpc^{-3}}$ at $z\simeq10$), matches the total SFRD inferred from observations within a factor of $<2$ at $7\lesssim z \lesssim10$. The Pop~III SFRD reaches a plateau at $\sim10^{-3}\ M_\odot{\rm yr^{-1}\, Mpc^{-3}}$ by $z~10$, and remains largely unaffected by the presence of Pop~II feedback. At $z$=7.5, $\sim20\%$ of Pop~III star formation occurs in dark matter haloes which are isolated, and 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 James Webb Space Telescope. We assess that our stellar mass function (SMF) and UV luminosity function (UVLF) agree well with the observed low mass/faint-end behaviour 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). Our measured turnover implies that observational studies which integrate their observed luminosity functions by extrapolating the observed faint-end slope beyond their detection limit may overestimate the true SFRD by a factor of $2 (10)$ when integrating to $M_{\rm UV} = -$12($-$8) at $z\sim10$. Our turnover correlates well with the transition from dark matter haloes dominated by molecular cooling to those dominated by atomic cooling, for a mass $M_{\rm halo}\approx10^{8}M_\odot$ at $z\simeq10$.