Legacy of star formation in the pre-reionization universe

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 $z\simeq 10$.