COOLING FLOWS IN HIERARCHICAL-CLUSTERING MODELS

In this paper we investigate the consequences of pregalactic and protogalactic cooling flows in the Cold Dark Matter and general hierarchical clustering models. We find that such flows are unlikely to process an appreciable fraction of the baryons in the Universe into low-mass stars in most models. This is because a significant amount of gas only cools quasi-statically in clouds for which the mean cooling time, t(c), and free-fall time, t(f), are comparable, whereas one expects most clouds to have t(c) < t(f) unless significant heating occurs as a result of massive star formation. The one exception is models dominated by non-baryonic 'warm' dark matter. Here the first objects to form satisfy t(c) approximately t(f), so that a substantial fraction of the gas in the Universe may cool quasi-statically. For other models we conclude that a less restrictive analogue of cluster cooling flows is required to provide a baryonic solution to the halo dark matter problem.