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We investigate the formation of molecular hydrogen (H₂_) in a primordial H + He gas cloud irradiated by a power-law UV flux. We find that at high densities (~>1 cm^-3^) and low temperatures (<~10⁴^ K), the background radiation enhances the formation of H₂_ and results in molecular cooling dominating over photoionization heating. This process could accelerate the collapse and fragmentation of dense objects at high redshifts.
Haiman et al. (Thu,) studied this question.