H-2 cooling of primordial gas triggered by UV irradiation

We investigate the formation of molecular hydrogen (H-2) in a primordial H + He gas cloud irradiated by a power-law UV flux. We find that at high densities (greater than or similar to 1 cm(-3)) and low temperatures(less than or similar to 10(4) K), the background radiation enhances the formation of H-2 and results in molecular cooling dominating over photoionization heating. This process could accelerate the collapse and fragmentation of dense objects at high redshifts.