Formation of Globular Clusters: In and Out of Dwarf Galaxies

Despite the rapid observational progress in the study of young massive star clusters, the formation of globular clusters still remains poorly understood. Yet, it is emerging that globular cluster formation is intimately linked to the formation of the Galaxy. I discuss a generic scenario of the cluster formation within progenitor galaxies, based on the available observational constraints. The oldest clusters formed around redshift z = 7, but the process continued at least until z = 3. Because of their high density, globular clusters survived when their progenitor galaxies were disrupted by the Galactic tidal field. 1 Observational Constraints Building a successful physical model of globular cluster formation has proved to be an elusive topic, as revealed by the multitude of effort and the lack of result in the last 30 years. In essence, this is the point where the large-scale galaxy formation problem meets the small-scale star formation problem. In order to try to overcome the uncertainties associated with both, we should seek as many clues as possible from observations. Fortunately, recent observational studies of star-forming regions in the Galaxy and young star clusters in interacting galaxies have improved our understanding of star formation. The majority of stars forms in clusters and associations of various sizes. The hierarchy ranges from the young OB associations to the old massive globular clusters, with no special scale between 10 and 10 M⊙: dN/dM ∝ M, α = 1.5 − 2 [3,19]. Thus globular clusters seem to represent the most massive end of the general hierarchy. The observed luminosity function (LF) differs from the initial one due to the dynamical evolution. Small-mass clusters (M < 10 M⊙) are gradually destroyed by stellar two-body relaxation and tidal interactions [18,5,15,6], while very massive clusters sink to the center via dynamical friction. However, most of the high-mass clusters are essentially unaffected by the evolution, and therefore, preserve the shape of the initial LF. Observations and modeling of old Galactic and extra-galactic globular clusters reveal similar general properties that depend little on the vastly different environment of the host galaxies (small and large, spiral and elliptical): • Age. The oldest globular clusters are old, but realistically old: tGC ≈ 12.5 Gyr, with the 95% probability range 10.2−15.9 Gyr [10]. There is no problem