Simulations of Pregalactic Structure Formation with Radiative Feedback

We present results from three-dimensional hydrodynamic simulations of the high-redshift collapse of pregalactic clouds including feedback effects from a soft H₂ photodissociating UV radiation field. The simulations use an Eulerian adaptive mesh refinement technique to follow the nonequilibrium chemistry of nine chemical species with cosmological initial conditions drawn from a popular Λ-dominated cold dark matter model. The results confirm that the soft UV background can delay the cooling and collapse of small halos (~10⁶ M_☉). For reasonable values of the photodissociating flux, the H₂ fraction is in equilibrium throughout most of the objects we simulate. We determine the mass threshold for collapse for a range of soft-UV fluxes and also derive a simple analytic expression. Continuing the simulations beyond the point of initial collapse demonstrates that the fraction of gas which can cool depends mostly on the virial mass of the halo and the amount of soft-UV flux, with remarkably little scatter. We parameterize this relation, for use in semianalytic models.