MEGATRON: The environments of Population III stars at Cosmic Dawn and their connection to present day galaxies

Abstract 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 halos with masses well below the atomic cooling limit, the majority of Pop III stars form in more massive systems above the 104 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 mini-halos, the LW background, gas starvation, and external chemical enrichment, the LW background is most important. A small fraction of halos undergo multiple episodes of Pop III star formation when the earlier forming stars all directly collapse to black holes. If the halos become massive enough, they can form up to ~100 Pop III stars in a single burst, which may be observable by JWST with moderate gravitational lensing. Pop III stars form at a wide range of distances from UV-bright galaxies, with only 0.06 % of Pop III stars forming within the virial radius of galaxies with MUV < −17. Finally, by tracking Pop III star remnants down to z = 0, we find that 75 − 80 % reside in the stellar halo of our simulated Milky Way analogue, while the remainder are gravitationally bound to lower-mass systems, including satellite halos.