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F. Schweizer (Carnegie Observatories)
Gravitational interactions and mergers are shaping and reshaping galaxies from the Local Group to the limits of the observable universe. While observations of interacting galaxies at redshifts z~\la 0.1 yield detailed information about the physical processes at work, observations at redshifts z~\ga 0.5 suggest that interactions and mergers were much more frequent in the past. Major mergers of nearby disk galaxies form remnants that share many properties with ellipticals and are, in essence, present-day protoellipticals. There is also growing evidence that minor mergers of companions contribute significantly to the formation of bulges in disk galaxies. Gas plays a crucial role in such interactions. Because of its dissipative nature, gas tends to get crunched into molecular form, turning into fuel for starbursts and active nuclei. Besides the direct evidence for ongoing interactions such as, e.g., in The Antennae galaxies, signatures of past interactions and mergers in galaxies abound: tidal tails and ripples, warped disks, counterrotating disks and bulges, polar rings, systems of young globular clusters, and aging starbursts. Clearly, galaxy formation and transformation is a prolonged process occurring to the present time. This includes the formation of elliptical and S0 galaxies, which we witness in mergers from z~\approx 0 -- 1. In essence, the observational evidence points towards Hubble's morphological sequence being a sequence of decreasing merger damage.
Research support from the National Science Foundation under grant AST-9900742 is gratefully acknowledged.