The Formation and Fragmentation of Primordial Molecular Clouds

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 IMF 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 0.5 pc are followed accurately. Dark matter dynamics, hydrodynamics and all relevant chemical and radiative processes (cooling) are followed self-consistently for a cluster normalized CDM structure formation model. Primordial molecular clouds with ~100,000 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 decreases the temperature to ~200 Kelvin in multiple "cold pockets" of the merger product. Within these cold pockets, quasi-hydrostatically contracting cores with masses \~ 100 M_sun and number densities > 100,000/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, our results indicate that the formation of very massive objects, massive black holes, fragmentation of a large fraction of baryons into brown dwars or Jupiter size fragments seems, in contrast to various claims in the literature, very unlikely. The expected escape fraction of UV photons with (h nu) > 11eV is very small.