  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 omega 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 protoelliptical galaxies with subpopulations of typically 100-1000 young
metal-rich globular clusters in their halos. The evidence is now strong that
these "second-generation" globular clusters formed from giant molecular clouds
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 giant molecular clouds squeezed by the
universal reionization pressure.
