Contribution of HD molecules in cooling of the primordial gas

Abstract

We study the effects of HD molecules on thermochemical evolution of the primordial gas behind shock waves, possibly arised in the process of galaxy formation. We find the critical shock velocity when deuterium transforms efficiently into HD molecules which then dominate gas cooling. Above this velocity the shocked gas is able to cool down to the temperature of the cosmic microwave background. Under these conditions the corresponding Jeans mass depends only on redshift and initial density of baryons $M_J \propto \delta_c^{-0.5} (1+z)^{0.5}$. At $z\simgt 45$ HD molecules heat shocked gas, and at larger redshift their contribution to thermal evolution becomes negligible.