In addition, it is well known that gas which cools to 1 eV through hydrogen line cooling will likely cool faster than it can recombine. This nonequilibrium cooling increases the number of electrons and ions (compared to the equilibrium case) which, in turn, increases the concentrations of and , the intermediaries that produce hydrogen molecules . If large concentrations of molecules form, excitations of the vibrational/rotational modes of the molecules can efficiently cool the gas to well below 1 eV, the minimum temperature expected from atomic hydrogen line cooling. Because the gas cools isobarically, the reduction in temperature results in an even greater reduction in the Jeans mass, and the bound objects which form from the fragmentation of cooled cosmological sheets may be associated with massive stars or star clusters. Anninos and Norman  have carried out 1D and 2D high resolution numerical calculations to investigate the role of hydrogen molecules in the cooling instability and fragmentation of cosmological sheets, considering the collapse of perturbation wavelengths from 1 Mpc to 10 Mpc. They find that for the more energetic (long wavelength) cases, the mass fraction of hydrogen molecules reaches , which cools the gas to and results in a fragmentation scale of . This represents reductions of 50 and 103 in temperature and Jeans mass respectively when compared, as in Figure 12, to the equivalent case in which hydrogen molecules were neglected.
However, the above calculations neglected important interactions arising from self-consistent treatments of radiation fields with ionizing and photo-dissociating photons and self-shielding effects. Susa and Umemura  studied the thermal history and hydrodynamical collapse of pancakes in a UV background radiation field. They solve the radiative transfer of photons together with the hydrodynamics and chemistry of atomic and molecular hydrogen species. Although their simulations were restricted to one-dimensional plane parallel symmetry, they suggest a classification scheme distinguishing different dynamical behavior and galaxy formation scenarios based on the UV background radiation level and a critical mass corresponding to density fluctuations in a standard CDM cosmology. These level parameters distinguish galaxy formation scenarios as they determine the local thermodynamics, the rate of line emissions and cooling, the amount of starburst activity, and the rate and mechanism of cloud collapse.
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