The Thermal Evolution of the Postshock Layer in Pregalactic Clouds

Abstract

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 ion­ ization 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 universe. Because of the coldness of the growing density perturbations, the formed clouds, which are the progenitors of galaxies, experience strong shock heating at the bouncing epoch. Shock heating is also expected in the hierarchical clustering scenario of structure formation. In this case, shocks are expected in the collision between two clouds which are trapped in the gravitational potential well associated with the larger structure. In any case, shock heating is expected in the era of galaxy formation. The spatial structure and the thermal evolution of the postshock layer in primordial gas clouds were investigated by many authors. 8), 26), 27),14),23),10),1),30) In those studies the common and most important point is the over production of hydrogen molecules. For example, in Shapiro and Kang 23) (hereafter SK), the thermal evolution of steady postshock flow is investigated. They found that the postshock flow cools down so fast that the recombination process cannot catch up with the cooling. As a result, the ionization degree remains high (Ye rv 10- 3 ), even when the temperature has dropped below 10 4 K. Feeded these relic electrons, hydrogen molecules form through the processes