The Intergalactic Medium: Initial and Boundary Conditions for Galaxy and Primeval Star Formation

Abstract

The formation of the first stars in gas of primordial composition depended upon the complex interplay of the hydrodynamics of gravitational collapse with nonequilibrium chemistry and its effect on the thermal evolution of the gas. The initial conditions and boundary values for the clouds which collapsed gravitationally to form galaxies and stars were determined by the thermal and ionization history of the general intergalactic medium. We describe new detailed numerical calculations of the evolution of the intergalactic medium in a post-recombination Friedmann universe, including a solution of the nonequilibrium rate equations for the ionization and recombination of H and He, the energy equation, and the equation of radiative transfer. The implications of this study for the characteristic mass scale and epoch of gravitational collapse for the average “first” star forming clouds will be discussed. The star formation rate and associated metallicity generation implied if the intergalactic medium was fully ionized by starlight by redshift z > 4 as suggested by recent interpretations of the Gunn-Peterson effect will also be discussed. Finally, we show that the relative strengths of metal lines from a quasar absorption-line Lyman limit system cloud at z = 3 photoionized by our intergalactic uv radiation background are compatible with the observations, at least for background sources of either AGN-type or star-forming galaxy spectra.