Formation of Globular Clusters in Merging Galaxies

Collisions and mergers of gas-rich galaxies trigger bursts of star and cluster formation. Of the thousands of clusters typically formed during a major merger, only the most massive and compact survive for Gigayears as globular clusters (GCs). In <~1 Gyr old merger remnants, these `second-generation' GCs appear by the hundreds as young halo clusters of about solar metallicity. Their likely descendants, metal-rich GCs of intermediate age (2 - 5 Gyr), have recently been found in ~10 elliptical galaxies, where they appear as slightly overluminous red GCs with a still power-law-like luminosity function. Their color and radial distributions suggest that they evolve into the red metal-rich GCs observed in old ellipticals. There is good evidence that second-generation GCs form from giant molecular clouds shocked by the rapid pressure increase in merger-induced starbursts. This mechanism supports the view that the universal pressure increase during cosmological reionization may have triggered the formation of the metal-poor globulars observed in galaxies of all types.