Globular Cluster Formation in Mergers

Abstract

Mergers of gas-rich galaxies lead to gravitationally driven increases in gas pressure that can trigger intense bursts of star and cluster formation. Although star formation itself is clustered, most newborn stellar aggregates are unbound associations and disperse. Gravitationally bound star clusters that survive for at least 10–20 internal crossing times (~20–40 Myr) are relatively rare and seem to contain 10% of all stars formed in the starbursts. The most massive young globular clusters formed in present-day mergers exceed ω Cen by an order of magnitude in mass, yet appear to have normal stellar initial mass functions.

In the local universe, recent remnants of major gas-rich disk mergers appear as proto-elliptical galaxies with subpopulations of typically 10²–10³ young metal-rich globular clusters in their halos. The evidence is now strong that these “second-generation” globular clusters formed from giant molecular clouds (GMC) in the merging disks, squeezed into collapse by large-scale shocks and high gas pressure rather than by high-velocity cloud–cloud collisions. Similarly, first-generation metal-poor globular clusters may have formed during cosmological reionization from low-metallicity GMCs squeezed by the universal reionization pressure.