The transition from population III to population II-I star formation: Early structures and critical metallicity

Abstract

We present results from the first cosmological simulations which study the onset of primordial, metal-free (population III), cosmic star formation and the transition to the present-day, metal-rich star formation (population II-I), including molecular (H2, HD, etc.) evolution, tracing the injection of metals by supernovæ (SNe) into the surrounding intergalactic medium and following the change in the initial mass function (IMF) according to the metallicity of the corresponding stellar population. Our investigation addresses the role of a wide variety of parameters (critical metallicity for the transition, IMF slope and range, SN/pair-instability SN metal yields, star formation threshold, resolution, etc.) on the metal-enrichment history and the associated transition in the star formation mode. All simulations present common trends. Metal enrichment is very patchy, with rare, unpolluted regions surviving at all redshifts, inducing the simultaneous presence of metal-free and metal-rich star formation regimes. As a result of the rapid pollution within high-density regions due to the first SN/pair-instability SN, local metallicity is quickly boosted above the critical metallicity for the transition. For this reason, population III stars dominate only during the very first stages of structure formation, with an average contribution to the total star formation rate that reaches a constant value of ∼10−3 at redshift ∼11–13. If primordial supenovæ consisted only of type II ones, the contribution would be ∼10−1. Interestingly, the above conclusions are independent from many poorly constrained parameters.