The interplay between chemical and mechanical feedback from the first generation of stars

Abstract

We study cosmological simulations of early structure formation, including non-equilibrium molecular chemistry, metal pollution from stellar evolution, transition from Population III (PopIII) to Population II (PopII) star formation, regulated by a given critical metallicity, and feedback effects. We perform analyses of the properties of the gas, and use the PopIII and PopII populations as tracers of the metallicity. This allows us to investigate the properties of early metal spreading from the different stellar populations and its interplay with pristine molecular gas, in terms of the initial mass function and critical metallicity. We find that, independently of the details about PopIII modelling, after the onset of star formation, regions enriched below the critical level are mostly found in isolated environments, while PopII star formation regions are much more clumped. Typical star-forming haloes, at z∼ 15–10, with masses between ∼107 and 108 M⊙, show average supernova (SN) driven outflow rates of up to ∼10−4 M⊙ yr−1 in enriched gas, initially leaving the original star formation regions almost devoid of metals. The polluted material, which is gravitationally incorporated in overdense environments on time-scales of ∼107 yr, is mostly coming from external, nearby star-forming sites (‘gravitational enrichment’). In parallel, the pristine-gas inflow rates are some orders of magnitudes larger, between ∼10−3 and 10−1 M⊙ yr−1. However, thermal feedback from SN destroys molecules within the pristine gas hindering its ability to cool and to condense into high-density star-forming regions. Only the polluted material incorporated via gravitational enrichment can continue to cool by atomic fine-structure transitions on short time-scales, short enough to end the initial PopIII regime within less than 108 yr. Moreover, the interplay between the pristine, cold, infalling gas and the ejected, hot, metal-rich gas leads to turbulent Reynolds numbers of the order of ∼108–1010, and contributes to the suppression of pristine inflow rates into the densest, star-forming regions.