Hydrogen molecules and the radiative cooling of pregalactic shocks

The nonequilibrium radiative cooling, recombination, and molecule formation behind steady state shock waves in a gas of primordial composition have been calculated in detail for a number of cases. The authors have solved the rate equations for these processes, together with the hydrodynamical conservation equations. Such shock waves are relevant to a wide range of theories of galaxy and pregalactic star formation. A purely atomic gas of H and He which is shock-heated to temperatures above 10⁴K is assumed. The results indicate that formation of H₂ molecules in the post-shock gas may be quite common for a significant range of shock velocities. The extra cooling resulting from H₂ formation greatly reduces previous estimates of the characteristic gravitational scale length and the characteristic mass subject to gravitational instability in these postshock regions.