Galaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxy interaction. Although galaxy mergers do not involve stars or star systems actually colliding, due to the vast distances between stars in most circumstances, the gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved. 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. There are some generally accepted results, however:

* When one of the galaxies is significantly larger than the other, the larger will often "eat" the smaller, absorbing most of its gas and stars with little other major effect on the larger galaxy. Our home galaxy, the Milky Way, is thought to be currently absorbing smaller galaxies in this fashion, such as the Canis Major Dwarf Galaxy, and possibly the Magellanic Clouds. The Virgo Stellar Stream is thought to be the remains of a dwarf galaxy that has been mostly merged with the Milky Way.

* If two spiral galaxies that are approximately the same size collide at appropriate angles and speeds, they will likely merge in a fashion that drives away much of the dust and gas through a variety of feedback mechanisms that often include a stage in which there are active galactic nuclei. This is thought to be the driving force behind many quasars. The end result is an elliptical galaxy, and many astronomers hypothesize that this is the primary mechanism that creates ellipticals.

Note that the Milky Way and the Andromeda Galaxy will probably collide in about 4.5 billion years. If these galaxies merged, the result would quite possibly be an elliptical galaxy as described above.

One of the largest galaxy mergers ever observed consisted of four elliptical galaxies in the cluster CL0958+4702. It may form one of the largest galaxies in the Universe.

Galaxy mergers can be simulated in computers, to learn more about galaxy formation. Galaxy pairs initially of any morphological type can be followed, taking into account all gravitational forces, and also the hydrodynamics and dissipation of the interstellar gas, the star formation out of the gas, and the energy and mass released back in the interstellar medium by supernovae.

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