Role of HD Molecules in the Cooling of the Primordial Gas

This paper examines the effect of HD molecules on the thermochemical evolution of the primordial gas behind the fronts of shock waves that may develop during the epoch of galaxy formation. A critical shock velocity is found at which deuterium efficiently converts to HD molecules and the contribution of HD molecules to cooling is dominant. Above this value of the shock velocity the gas can cool to temperatures close to that of the cosmic background radiation. In this case the Jeans mass will depend only on the red shift and the initial density, with MJ ∝ δ c−0.5 (1 + z)0.5. For z≳45, HD molecules heat the gas and for large red shifts they generally cease to play a significant role in the thermal evolution of the gas.