Cosmological Formation of Low-Mass Objects

We investigate the early formation of bound objects with masses comparable to the cosmological Jeans mass (~10⁵^ Mₛun_). We follow the growth of isolated spherically symmetric density peaks starting from the linear perturbative regime. The initial parameters correspond to density peaks of various widths and heights in a cold dark matter cosmology. We use a one-dimensional spherical Lagrangian hydrodynamics code to follow the dynamical, thermal, and nonequilibrium chemical evolution of the gas. The system includes a collisionless dark matter component and a baryonic component composed of the nine species H, H^-^, H^+^, He, He^+^, He^++^, H₂_, H₂_^+^, and e^-^. All relevant chemical reactions between these species and their cooling mechanisms are included in the calculations. We explore the dependence of the dynamical evolution of the gas on two parameters: the initial mass scale and the initial overdensity of the system. We follow the evolution of the density, temperature, and abundance profiles within the cloud, assuming two types of central boundary conditions for the collisionless component: in one the infalling dark matter virializes through a reflection from a hard sphere, while in the other it accretes onto a central sink. We find that in both cases, radiative cooling by H₂_ affects the collapse dynamics of the gas only after it has already virialized and become part of the bound object. Therefore, radiative cooling is unlikely to have triggered the initial collapse of perturbations at redshifts z > 10. Nevertheless, baryonic objects with masses well below the linear theory Jeans mass (<~10³^ Mₛun_) form at high redshifts because of shell crossing by the dark matter. Such objects could be the progenitors of a primordial population of high-mass stars in the intergalactic medium.