This review has high-lighted the role that interactions and mergers
play in driving galaxy evolution. At present we remain challenged to
understand the relative importance of weak and strong interactions, the
details of bulge formation, the existence of nearly pure-disk galaxies,
and the merger rate as a function of redshift. Yet, some firm conclusions
have been reached and are as follows:
- Gravitational interactions and mergers are forming and transforming
galaxies throughout the observable universe. The vast majority involve
gas, dissipation, and enhanced star formation.
- The close link between mergers, ultra-luminous infrared galaxies,
and quasars suggests that - like quasar activity - major merging may have
peaked around z 2.
- Major disk-disk mergers form elliptical galaxies with kinematic
subsystems, bimodal globular-cluster populations, and remnant fine structure.
Such mergers occurred relatively early near the centers of rich clusters,
but continue to the present time in rich-cluster outskirts, poorer clusters,
and the field.
- Minor mergers tend to move disk galaxies toward earlier morphological
types, creating kinematic subsystems and some bulges (fraction remains
- In short, the currently available evidence strongly suggests that
Hubble's morphological sequence is mainly a sequence of decreasing merger
I gratefully acknowledge research support from the Carnegie Institution of
Washington and from the National Science Foundation under Grant AST-99 00742.