The term galaxy merger is used to designate the act of two or more galaxies becoming one when colliding. Stars generally do not collide, but gas and dust clouds are affected.
It is an example of a galaxy interaction, which also includes the effect of one galaxy on another just from being near enough to affect the other's gravitational field. Such close encounters can modify the shape of a galaxy, and even pull some portions of the galaxy away, e.g., tidal arms.
Mergers are thought to boost star formation, specifically when gas and dust from the galaxies collide. Simulations suggest the increase in star formation rate is greatest when the galaxies have similar mass, i.e., a small Galaxy Merger Mass Ratio.
Sometimes galaxy collisions can be clearly seen or seen to be inevitable. Merging or partial merging can be seen in trails of stars between the collided galaxies. Galactic asymmetry is a sign of a possible recent merger. A galaxy with nuclei is often a sign of a merger.
It is thought that mergers can trigger an active galactic nucleus.
Evidence of past mergers is observed in nearby galaxies, leading to the idea that mergers are common in galaxy growth. Elliptical galaxies are theorized to be a product of mergers, to the point where orderly concentric orbits have been randomized.
The galaxy merger timescale is the timescale of the duration of the merging process.
One classification of galaxy mergers is major (two galaxies of similar size, e.g., spiral galaxies, which might form an elliptical galaxy) versus minor (a small galaxy joining a large galaxy). Mergers may also be classified as local, e.g., using a cutoff at a redshift of 0.1.