Abstract
ABSmACT. 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 nuinber of cases. We have solved the rate equations for these processes, together with the hydrodynamical conservation equations. Shock waves such as these are relevant to a wide range of theories of galaxy and pregalactic star formation. We have described elsewhere our calculations for shock velocities ranging from 50 to 400 km 5.1 for shocks occurring in pregalactic gas at redshifts z =5, 10 and 20. We present results here for a different range of cases, focusing on shocks of relatively low velocity (v =20,30,50 km s.1) occurring at high redshift (z =20,100). Such shocks may occur, for example, in primordial cloud-cloud collisions, in the gravitational collapse of cosmological density fluctuations of subgalactic mass, and in the wakes of cosmological strings. A purely atomic gas of H and He which is shock- heated to temperatures above l(AK and is assumed to remain in ionization equilibrium as it cools in the postshock flow will not in general be able to cool radiatively to temperatures much below . When proper account is taken of departures from ionization equilibrium, however, the nonequilibrium recombination which occurs as such a gas cools makes possible H2 formation which can enable the gas to cool to much lower temperatures. At redshifts as low as z = 5, the low velocity shocks considered here do not generally form enough H2 rapidly enough to cool the postshock gas to l K within a Hubble time, in contrast to our previous results for higher velocity shocks. Our results indicate that at high redshift, however, even for shock velocities as low as this, H2 molecules can form in the postshock gas with concentrations l0- sufficient to cool the gas to 102K within a time comparable to the age of the universe at the redshift of shock-heating. This extra cooling from 104K to 102K at nearly constant pressure greatly reduces the characteristic gravitational scale length in the cooled gas. The spherical Jeans length, for example, is reduced by two orders of magnitude. This has important implications for theories involving primordial star formation at high redshift.
