The Formation and Fragmentation of Primordial Molecular Clouds

TLDR

The thermal history, chemical enrichment, and reionization of the intergalactic medium are closely tied to the formation and evolution of pregalactic structures, making the efficiency of primordial star formation and the initial mass function of particular interest. We use high‑resolution three‑dimensional adaptive mesh refinement simulations to follow the collapse and fragmentation of primordial molecular clouds in a cosmologically representative volume. The simulations track comoving scales from 128 kpc down to 1 pc, self‑consistently modeling dark‑matter dynamics, hydrodynamics, and all relevant chemical and radiative processes in a cluster‑normalized cold‑dark‑matter cosmology, assembling ≈10^5 M⊙ primordial clouds through mergers of H₂‑rich objects with fractional abundances ≲10⁻⁴. Cooling in the merged halos produces a ~200 K cold pocket that hosts a quasi‑hydrostatic core of ≈200 M⊙ and densities ≳10⁵ cm⁻³, yet less than 1 % of the primordial gas reaches such high densities, and the study sets a new record for dynamic range in cosmological AMR hydrodynamics.

Abstract

Many questions in physical cosmology regarding the thermal history of the intergalactic medium, chemical enrichment, reionization, etc., are thought to be intimately related to the nature and evolution of pregalactic structure. In particular, the efficiency of primordial star formation and the primordial initial mass function are of special interest. We present results from high-resolution three-dimensional adaptive mesh refinement simulations that follow the collapse of primordial molecular clouds and their subsequent fragmentation within a cosmologically representative volume. Comoving scales from 128 kpc down to 1 pc are followed accurately. Dark matter dynamics, hydrodynamics, and all relevant chemical and radiative processes (cooling) are followed self-consistently for a cluster-normalized cold dark matter (CDM) structure formation model. Primordial molecular clouds with ~105 solar masses are assembled by mergers of multiple objects that have formed hydrogen molecules in the gas phase with a fractional abundance of ≲10-4. As the subclumps merge, cooling lowers the temperature to ~200 K in a "cold pocket" at the center of the halo. Within this cold pocket, a quasi-hydrostatically contracting core with mass ~200 M☉ and number densities ≳105 cm-3 are found. We find that less than 1% of the primordial gas in such small-scale structures cools and collapses to sufficiently high densities to be available for primordial star formation. Furthermore, it is worthwhile to note that this study achieved the highest dynamic range covered by structured adaptive mesh techniques in cosmological hydrodynamics to date.