Globular Clusters and Dark Clusters

Abstract

Fall and Rees have suggested that thermal instability in the collapsing gas of a protogalaxy gives rise to cool clouds embedded in a hot medium. They argue that the temperature of the clouds cannot fall below 10⁴K, since metals and molecular coolants are absent. Clouds with masses exceeding 10⁶M_⊙ are gravitationally unstable and are identified as the precursors of globular clusters. This model has difficulty in explaining high-metallicity globular clusters, since metals provide cooling down to ∼10²K or below, thus considerably reducing the cloud Jeans mass. The same problem arises if H₂ cooling occurs.