Radiative Shocks and Nonequilibrium Chemistry in the Early Universe: Galaxy and Primordial Star Formation
Shock waves in primordial composition gas occur in a wide range of circumstances in the theory of galaxy and pregalactic star formation. The radiative cooling of the postshock gas is crucial to the successful production of gravitationaily bound fragments. Without such fragmentation, the models do not form stars and galaxies. We have studied in detail the nonequilibrium radiative cooling, recombination, and molecule formation behind steady-state shock waves in primordial composition gas. We have solved the hydrodynamical conservation equations, along with the rate equations fo nonequilibrium ionization, recombination, and molecule formation and the equation of radiative transfer. We find that the shocked gas cools faster than it can recombine and, as a result, is able to form an H2 concentration as high as 10−3 or higher via the formation of H− and H 2 + intermediaries due to the enhanced nonequilibrium ionization at 104 K. With such an H2 concentration, the gas cools by rotational-vibrational line excitation of H2 molecules to well below the canonical final temperature of 104 K for a molecule-free gas without metals. This cooling below 104 K significantly lowers the characteristic gravitationaily unstable mass estimated for shocks relative to the value if the gas cooling stops at 104 K. We show that, as the level of external ionizing and dissociating radiation flux is increased, the formation of and cooling by H2 molecules can be inhibited and delayed. In addition to their relevance to the pregalactic and intergalactic medium, shocks such as these may be responsible for the formation of globular clusters inside protogalaxies. The implications of our shock calculations for this model of the origin of globular clusters will also be discussed.
KeywordsActive Galactic Nucleus Globular Cluster Radiative Cool Gravitational Instability Radiative Shock
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