The thermal balance of the first structures in the primordial gas

The contraction of matter in the primordial medium, to form the first gravitationally bound structures, was mediated by radiative cooling of the gas by H₂ and HD. We have computed the initial phases of free-fall collapse, incorporating the results of quantum mechanical calculations of rate coefficients for collisional excitation of H₂ and HD by the principal perturbers, H, He, H₂ and H⁺. The structure of shock waves produced when the collapse speed exceeds the local sound speed is determined. In the post-shock gas, radiative cooling by H₂ exceeds that by HD, but by a factor of only 4. The intensities of the strongest emission lines of H₂– rotational transitions within the vibrational ground state – are calculated. Even with coarse spectral and angular resolution, these transitions might be observable as inhomogeneities in the cosmic background radiation.