Oscar Agertz

We present results of Population III (Pop III) formation in the megatron suite of simulations, which self-consistently follows radiation and non-equilibrium chemistry, and resolves gas at near-pc resolution in a Milky Way-mass progenitor at Cosmic Dawn. While the very first Pop III stars form in haloes with masses well below the atomic cooling limit, the majority of Pop III stars form in more massive systems above the $10^4$ K atomic cooling threshold as a Lyman–Werner (LW) background of $10^{-21}\, \rm erg\, s^{-1}\, cm^{-2}\, Hz^{-1}\, sr^{-1}$ is rapidly established. We find that the global Pop III star formation rate stabilizes to a value of $10^{-3}\, \rm M_\odot \, yr^{-1}$ at $z=20$. Among the three processes that quench Pop III star formation in minihaloes, the LW background, gas starvation, and external chemical enrichment, the LW background is most important. A small fraction of haloes undergo multiple episodes of Pop III star formation when the earlier forming stars all directly collapse to black holes. If the haloes become massive enough, they can form up to $\sim 100$ Pop III stars in a single burst, which may be observable by James Webb Space Telescope with moderate gravitational lensing. Pop III stars form at a wide range of distances from UV-bright galaxies, with only $0.06\,$ per cent of Pop III stars forming within the virial radius of galaxies with $M_{\rm UV}\lt -17$. Finally, by tracking Pop III star remnants down to $z=0$, we find that $75-80\,$ per cent reside in the stellar halo of our simulated Milky Way analogue, while the remainder are gravitationally bound to lower mass systems, including satellite haloes.