A Universal Formation Mechanism for Open and Globular Clusters in Turbulent Gas

Abstract

A universal mechanism for cluster formation in all epochs and environments is found to be consistent with the properties and locations of young and old globular clusters, open clusters and unbound associations, and interstellar clouds. The primary structural differences between various cluster types result from differences in pressure at the time of formation, combined with different ages for subsequent evolution. All clusters begin with a mass distribution similar to that for interstellar clouds, which is approximately n ( M ) dM ∝ M -2 dM . Old halo globulars have a current mass distribution that falls off at low mass because of a Hubble time of cluster destruction. Young globulars have not yet had time for a similar loss, and some old open clusters have survived because of their low densities. The peak in the halo cluster luminosity function depends only on age, and is independent of the host galaxy luminosity, as observed. The peak globular cluster mass is not a characteristic or Jeans mass in the primordial galaxy, as previously suggested. The initial mass distribution functions for young and old globular clusters, open clusters and associations, and interstellar clouds are all power laws with a slope of ~ -2. This distribution could be the result of fractal structure in turbulent gas. New data on clusters in the LMC also follow this power law. The slope is so steep that it implies a significant fraction of star formation occurs in small clusters. Numerous halo field stars should come from the evaporation of small halo clusters, and a high fraction of disk field stars should arise in small unbound disk clusters. This differs significantly from previous suggestions that most disk stars form in large OB associations. Globular clusters of all ages preferentially form in high-pressure regions. This is directly evident today in the form of large kinematic pressures from the densities and relative velocities of member stars. High pressures at the time of globular cluster formation are either the result of a high background virial density in that part of the galaxy (as in dwarf galaxies or galactic nuclei and nuclear rings), turbulence compression (in halo globulars), or large-scale shocks (in interacting galaxies). Massive clusters that form in such high-pressure environments are more likely to be bound than low-mass clusters or clusters of equal mass in low-pressure regions. This is because virialized clouds are more tightly bound at high pressure. A simple model illustrates this effect. One implication of this result is that starburst regions preferentially make globular clusters, in which case some elliptical galaxies could have formed by the violent merger of spiral galaxies.