galaxy merger

The term galaxy merger refers to two or more galaxies becoming one, which can happen if they collide. Their stars generally do not collide with each other, but gas and dust clouds are affected. Such a merger is a type of galaxy interaction, other types of interactions corresponding to lesser effects, such as their gravitational interactions if they pass close to each other but don't collide. Such close encounters can modify the shape of a galaxy, and even pull some portions of the galaxy away, e.g., tidal arms. Actual 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 galaxy merger mass ratio close to 1. It is thought that mergers can trigger an active galactic nucleus (AGN).

In some cases, a galaxy collision is clearly evident or can be seen to be inevitable. Merging or partial merging can be suggested by trails of stars between the collided galaxies (tidal tails). Galactic asymmetry is a sign of a possible recent merger. A galaxy with two (or more) nuclei is often a sign of a merger. Evidence of past mergers is observed in galaxies near to ours, i.e., those in which we can observe the most detail, which suggests mergers are common and likely a common means of galaxy growth. Elliptical galaxies are theorized to be a product of mergers, having reached 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. The term galaxy cannibalism is also used for the latter, perhaps when the size-difference is extreme. A double minor merger is two happening one right after the other (e.g., within 100 million years), a scenario that has been explored regarding galaxy quenching. Mergers may also be classified as local, e.g., using a cutoff at a redshift of 0.1.

The term peculiar galaxy includes apparent galaxy mergers in progress, galaxies strongly affected by lesser interactions (disturbed galaxies), and other sources of oddness, such as some AGNs. The Toomre sequence is a selected list of galaxy-interaction images that demonstrate the stages of a merger.

The Milky Way and Andromeda are currently considerably separated, but given their trajectories, they will collide in a few gigayears and merge. The Milky Way is presumed to have merged with dwarf galaxies in the past, and like galaxies in general, is thought to be the product of such mergers.