Galaxy mergers can occur when two galaxies collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved, but the exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. Galaxy mergers are important because the merger rate is a fundamental measurement of galaxy evolution and also provides astronomers with clues about how galaxies grew into their current forms over long stretches of time.
The Mice Galaxies (NGC 4676 A&B) are in the process of merging.
NGC 3921 is an interacting pair of disc galaxies in the late stages of its merger.
Arp 302 (left); NGC 7752/7753; IIZw96 (right).
Galaxy twistings – possible merger.
Interacting galaxies are galaxies whose gravitational fields result in a disturbance of one another. An example of a minor interaction is a satellite galaxy disturbing the primary galaxy's spiral arms. An example of a major interaction is a galactic collision, which may lead to a galaxy merger.
NGC 3169 (left) and NGC 3166 (right) display some curious features, showing that each is close enough to feel the distorting gravitational influence of the other. Image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at the La Silla Observatory.
Gravitationally lensed galactic merger H-ATLAS J142935.3-002836.
UGC 2369 taken by HST.
Close encounter at IRAS 06076-2139.