Abstract

We use cosmological simulations to study the origin of primordial star-forming in a ΛCDM universe, by following the formation of dark matter halos and the cooling of within them. To model the physics of chemically pristine gas, we employ a nonequilibrium treatment of the chemistry of nine species (e−, H, H+, He, He+, He++, H2, H, H−) and include cooling by molecular hydrogen. By considering cosmological volumes, we are able to study the statistical properties of primordial halos, and the high resolution of our simulations enables us to examine these objects in detail. In particular, we explore the hierarchical growth of bound structures forming at redshifts z ≈ 25-30 with total masses in the range ≈105-106 M☉. We find that when the amount of molecular hydrogen in these objects reaches a critical level, cooling by rotational line emission is efficient, and dense clumps of cold form. We identify these gas clouds as sites for primordial star formation. In our simulations, the threshold for cloud formation by molecular cooling corresponds to a critical halo mass of ≈5 × 105 h-1 M☉, in agreement with earlier estimates, but with a weak dependence on redshift in the range z > 16. The complex interplay between the gravitational formation of dark halos and the thermodynamic and chemical evolution of the compromises analytic estimates of the critical H2 fraction. Dynamical heating from mass accretion and mergers opposes relatively inefficient cooling by molecular hydrogen, delaying the production of star-forming in rapidly growing halos. We also investigate the effect of photodissociating ultraviolet radiation on the formation of primordial clouds. We consider two extreme cases, first by including a uniform radiation field in the optically thin limit and second by accounting for the maximum effect of self-shielding in virialized regions. For radiation with Lyman-Werner band flux J > 10-23 ergs s-1 cm-2 Hz-1 sr-1, hydrogen molecules are rapidly dissociated, rendering cooling inefficient. In both the cases we consider, the overall effect can be described by computing an equilibrium H2 abundance for the radiation flux and defining an effective shielding factor. Based on our numerical results, we develop a semianalytic model of the formation of the first stars and demonstrate how it can be coupled with large N-body simulations to predict the star formation rate in the early universe.

Highlights

  • The first stars in the Universe almost certainly originated under conditions rather different from those characterizing present-day star formation

  • We investigate the impact of photo-dissociating ultra-violet (UV) radiation on the formation of primordial gas clouds

  • Based on our numerical results, we develop a semi-analytic model of the formation of the first stars, and demonstrate how it can be coupled with large N -body simulations to predict the star formation rate in the early universe

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Summary

INTRODUCTION

The first stars in the Universe almost certainly originated under conditions rather different from those characterizing present-day star formation. Numerical studies of the formation of primordial gas clouds and the first stars indicate that this process likely began as early as z ≈ 30 (Abel, Bryan & Norman 2002; Bromm, Coppi & Larson 2002) In these simulations, dense, cold clouds of self-gravitating molecular gas develop in the inner regions of small halos and contract into proto-stellar objects with masses in the range ≈ 100 − 1000M⊙. The first in a series on early structure formation, we study the cooling and collapse of primordial gas in dark halos using high resolution cosmological simulations. The cooled dense gas clouds appear as bright spots at the intersections of the filamentary structures

THE MINIMUM COLLAPSE MASS
HALO FORMATION HISTORY
THE ANGULAR MOMENTA OF DARK MATTER AND GAS
GAS CLOUD EVOLUTION
EFFECT OF RADIATIVE FEEDBACK
Uniform background radiation
Self-shielding
Minimum collapse mass under far UV radiation
Processes neglected
SEMI-ANALYTIC MODELING OF THE EARLY STAR FORMATION
Building up the cosmic UV background
Gas cloud formation
Pop III star formation at high redshift
The cosmic star formation history
DISCUSSIONS
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