ON THE FRAGMENTATION OF PROTOGALACTIC CLOUDS

After its initial collapse, a protogalactic cloud is expected to be shock heated to its virial temperature of ∼ 106 K. This temperature lies near a very unstable region of the cooling curve, so that density enhancements will be expected to grow rapidly as the protogalaxy cools to lower temperatures. In this contribution, the dynamical effects on the growth of perturbations are examined. We show that perturbations with large initial amplitudes lead to the formation of H2, which results in rapid cooling to 100 K, and the formation of Population III stars. Perturbations with small initial amplitudes, however, do not cool significantly ahead of the background, do not develop strong contrasts with the background, and so cannot induce the formation of H2. They thus remain near 104 K, forming a population of protoclusters with masses ∼107 M⊙, somewhat larger than typical globular cluster masses. Contamination by metals from a primordial generation of stars may lead to cooling and a significant reduction of the Jeans mass. Photodissociation induced by a background source of hard photons (≳1043 ergs s-1) can also prevent cooling below 104 K, enhancing the formation of protoclusters relative to stars.