The Thermal Evolution of the Postshock Layer in Pregalactic Clouds

We re-examine the thermal evolution of the postshock layer in primordial gas clouds. Comparing the time scales, we find that the evolutionary paths of postshock regions in primordial gas clouds can be basically understood in terms of the diagram drawn in the ionization degree vs temperature plane. The results obtained from the diagram are independent of the density in the case that we do not include photodissociation and photoionization. We also argue that the diagram is not only relevant to the case of the steady postshock flow, but also to the isochorically cooling gas. The thermal evolution of primordial gas clouds has been investigated by many authors. 15), 24), 3), 17), 9), 20), 29) Almost all of those studies have been concerned with the formation of various kinds of galaxies, or primordial stars. In those papers, galaxies are assumed to grow out of small density perturbations present in the early