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Galaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxy interaction. Stars in these galaxies are extremely unlikely to collide due to the vastness of interstellar distances. Gas clouds may collide during the merger, however, generating powerful shocks and leading to star formation.

While only 1-2% of luminous galaxies are experiencing current mergers, almost all galaxies have been shaped by interactions in the past. The rate of galaxy collisions appears to have been higher in the early Universe. .

Basic Physics
Gravitational effects such as dynamical friction and tidal disruption have major effects on the galaxies involved. Dynamical friction is a drag force that can slow galaxies and lead to a merger. Dynamical friction results from the gravitational forces between the stars in the merging galaxies. As one galaxy (Galaxy A) moves past stars in another galaxy (Galaxy B), the stars in B are attracted to Galaxy A by its gravity and accumulate behind it. The gravity of these stars acts as a braking force and slows Galaxy A, and the stars gain kinetic energy at the expense of the galaxy. If the two galaxies are moving slowly relative to each other, they will have more time to exchange energy, their deceleration will be greater, and the interaction will have stronger effects. If the galaxies lose enough energy, they will merge to form one system.

Tidal disruption is a result of galactic tides. When one galaxy is close to another, the near side of the galaxy will feel a stronger gravitational force than the rest of the galaxy. This difference in force stretches out the galaxy and is responsible for the formation of structures such as tidal tails, which have been observed in many merging systems.

Types of Mergers
The exact effects of mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition. A major merger is defined as a merger between two galaxies of similar size, while a minor merger refers to a merger between two galaxies with very different sizes. For example, the merger of a large galaxy with a small satellite galaxy would be considered a minor merger. The precise definition of what constitutes a major merger is unclear, but mass ratios of above 1:2 or 1:3 are generally accepted. In a minor merger, the larger galaxy will often "eat" the smaller galaxy, 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.

‘Wet’ mergers are mergers between two gas-rich, spiral galaxies, while in ‘dry’ mergers, both galaxies have little to no gas. ‘Mixed’ mergers involve one gas-rich and one gas-poor galaxy.

Spatial distribution and structure
The simplest method of identifying merging systems is to identify pairs of galaxies that appear close to one another on the sky. Visually close pairs of galaxies are not necessarily physically associated with one another, however, and may simply lie along the same line of sight. Redshift information can be used to determine whether the galaxies are truly close to each other. Identifying close pairs may be a useful method of finding galaxies in the pre-merger stage. Galaxies' morphologies often reveal signs of ongoing or past merger activity. Major mergers can result in large-scale changes in a galaxy's morphology, while in minor mergers, the basic structure of the primary galaxy is unaltered. Sometimes double nuclei or double disk structures are visible. These structures are not in equilibrium and indicate a recent merger. Elliptical galaxies typically contain little dust; the presence of a dust lane in an elliptical galaxy may be evidence of a major merger in the galaxy's past. One of the most distinctive features in images of merging galaxies are tidal tails. Stars can gain substantial angular momentum and energy from the merging galaxy, causing them to move out to large distances from their host galaxy. Asymmetric structures such as tidal tails and shells of material are often used to identify ongoing or past mergers. Ring galaxies are also suspected to form through mergers. They likely result from a major, head-on merger involving at least one disk galaxy.

Kinematics
The motions of stars and gas in a galaxy also provide a sign of past interactions and are especially useful in identifying minor mergers. When stars and gas gain energy and angular momentum at the expense of the other galaxy, their orbits become more elongated. Galaxies that have undergone a merger may have a population of stars and gas that rotates around a different axis from the rest of the galaxy or with highly inclined orbits. Some post-merger galaxies may have gas disks that do not rotate at the same rate as the stars in the galaxy or may even counter-rotate with respect to the stars. Star formation during the merger may result in a dense grouping of stars near the galaxy center that rotates around a different axis from the older stars in the galaxy. This phenomenon is known as a kinematically decoupled core.

Galaxy assembly
The speed of light is limited, so it takes the light from more distant objects more time to reach the observer. By looking deep into the sky, astronomers are actually looking back in time, seeing the Universe as it was billions of years ago. Pictures from the Hubble Ultra Deep Field show a snapshot of the young Universe, in which furious interactions between small galaxies took place. Careful studies of the cosmic microwave background radiation show that the early Universe was not completely uniform; there were clumps in the distribution of mass and temperature. Galaxies are now thought to have formed in a "bottom up" fashion; small components formed in denser regions of the early Universe. Through interactions with each other, these small components merged into larger components. Over time, collision and merging continued to shape these progenitors to form the galaxies and clusters in our present-day Universe.

Galaxy structure
Spiral galaxies have spiral arms, a central bulge structure, and sometimes a central bar. In addition to the density wave and Self-propagating star formation models of spiral arm formation, simulations demonstrate that close encounters of galaxies can also produce spiral arms. During collisions or disruptions from another close galaxy, the interstellar medium is compressed, triggering star bursts, and streams of stars can be stretched out, producing the spiral arms. Simulations also suggest that a central bar structure can be produced from perturbations due to galaxy interactions. This suggestion is supported by observations which show that bars are more likely to be found in close pairs of galaxies. However, further exploration of this environmentally-triggered bar formation has not yet been done. The bulges of galaxies are either classical bulges or disk-like bulges. Classical bulges resemble elliptical galaxies and are thought to be built by merging, while disk-like bulges are formed through secular processes.

Lenticular galaxies are galaxies that are morphologically intermediate between elliptical galaxies and spiral galaxies, they are classified as S0 galaxies in Hubble sequence. They are disk-like, arm-less, and have a smooth appearance and low star formation rate similar to elliptical galaxies. As their appearance suggests, lenticular galaxies may come from faded spiral galaxies, whose arm features disappeared. However, lenticular galaxies are likely to be more luminous than other spiral galaxies, which suggests that they are not merely the remnants of dead spiral galaxies. Instead, lenticular galaxies might result from mergers, which increase the total stellar mass and give the newly-merged galaxy its disk-like and arm-less appearance.

Coreless ellipticals have a central light component that is typically disk-like. This disk-like component suggests that if these galaxies form from mergers, the mergers must be 'wet'. Massive elliptical galaxies tend to be slower-rotating, more axisymmetric, and have a less cuspy center. Major dry mergers may play an important role in their formation, but minor dry mergers likely do not. 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.

Star formation
Enhanced star formation rates are often associated with mergers, and many starburst galaxies are associated with merging or closely interacting systems. Almost all of the known Ultraluminous Infrared Galaxies (ULIRGs) are merging systems, where both galaxies are massive, gas-rich spirals. Not all mergers result in starbursts, however, and it is not known why some mergers lead to starbursts, while others do not. The mergers that do lead to enhanced star formation must have a sufficient gas supply; dry mergers will not result in a starburst. Mixed mergers may lead to starbursts, but there is generally less star formation than for wet mergers.

If the merging galaxies do contain a large amount of gas, the merger process can funnel that gas to the center of the merged galaxy and power a starburst. Rotationally-supported systems like disk galaxies can lose angular momentum from gravitational torques and due to energy dissipation in shocks. As the gas loses angular momentum, it flows to the center of the system, resulting in a higher central gas density and a nuclear starburst, a burst of star formation near the galactic nucleus. The merger disrupts the gas' orbits; collisions of gas clouds throughout the galaxy compress the gas and may provide another means of inducing star formation. Mergers may also contribute to the formation of galactic bars, which provide non-axisymmetric gravitational forces that can drive gas inwards and fuel star formation.

Brightest cluster galaxy
Brightest cluster galaxies (BCGs) are a special class of elliptical galaxies. They are usually the brightest member in a massive cluster and are wrapped by an extending distribution of stars, called a cD envelope (or intracluster light, ICL). Most of the BCGs are larger than the radius-luminosity relationship for elliptical galaxies would predict. Their fundamental plane relation also differs from other ellipticals, and their cD envelope is inconsistent with the intensity profile of a typical galaxy. Most significantly, there is a magnitude gap between BCGs and the second-brightest cluster galaxies that cannot be explained by a random sampling of the galaxy luminosity function. All of these features suggest BCGs are not merely a brighter, more massive class of ellipticals. The formation of BCGs could be explained by dry mergers. Dynamical friction increases as the the cluster mass increases, slows down and draws the passing galaxies into the cluster center where the most massive elliptical galaxy is located. Any second-ranked galaxy with comparable mass to the BCG would be accreted by the BCG first, further expanding the magnitude gap as observed.

Mergers in external galaxies
Some galaxies that are suspected to be in the process of merging:
 * The Antennae Galaxies/NGC4038 & NGC 4039 - These galaxies show tidal tails and have starbursts.
 * The Mice Galaxies/NGC 4676 - Galaxies known for their tidal tails.
 * Centaurus A - A starburst system which has a lenticular morphology with a prominent dust lane.
 * NGC 7318 - Galaxies in the famous group Stephan's Quintet. Tidal tails and enhanced star formation are visible.
 * Arp 220 - An Ultraluminous infrared galaxy.
 * Many additional famous examples are identified in the Atlas of Peculiar Galaxies compiled by Halton Arp.
 * The Cartwheel Galaxy - A famous ring galaxy.
 * CL0958+4702 - This galaxy cluster contains one of the largest galaxy mergers ever observed, consisting of four elliptical galaxies. It may form one of the largest galaxies in the Universe.

Merger remnants in the Milky Way

 * Sagittarius Dwarf Elliptical Galaxy - This dwarf galaxy is in the process of merging with the Milky Way.
 * Virgo Stellar Stream - The remnant of a dwarf spheroidal galaxy merging with the Milky Way.
 * Canis Major Dwarf Galaxy - This dwarf galaxy has experienced significant tidal disruption and its stars encircle the Milky Way.

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.

Computer simulations
While observations provide a snapshot of the merger process, animations from supercomputers help us visualize the entire merger process on a million year time-scale. 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. Such a library of galaxy merger simulations can be found on the GALMER website

We can explore whether a particular structure could form by changing the orientation of colliding galaxies or their types and masses. Simulations provide us another means of understanding observed merger mechanisms. The earliest simulation of galaxy mergers was done by Toomre & Toomre in 1970s. They found that the bridges and tails in multiple galaxies are actually the remnants of close encounters. Various codes for astrophysical simulations have been developed, and different computation strategies can be used depending on the physical conditions of a particular question. For example, studies on star formation in galaxy mergers have to couple two extreme resolution needs. Details of star formation need to be resolved in a parsec (pc) scale, while the tidal interactions of mergers need a Megaparsec (Mpc) scale computation volume. Scientists thus use an adaptive mesh refinement (AMR) technique to track the interesting region, avoiding wasting computation time over the entire computation volume.