Galaxy mergers can occur when two (or more) 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. 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 fundamental measurement of galaxy evolution. The merger rate also provides astronomers with clues about how galaxies bulked up over time.
During the merger, stars and dark matter in each galaxy become affected by the approaching galaxy. Toward the late stages of the merger, the gravitational potential (i.e. the shape of the galaxy) begins changing so quickly that star orbits are greatly affected, and lose any memory of their previous orbit. This process is called violent relaxation. Thus if two disk galaxies collide, they begin with their stars in an orderly rotation in the plane of the disk. During the merger, the ordered motion is transformed into random energy. The resultant galaxy is dominated by stars that orbit the galaxy in a complex, and random, web of orbits. This is what we see in elliptical galaxies, stars on random unordered orbits.
Mergers are also locations of extreme amounts of star formation. The star formation rate (SFR) during a major merger can reach thousands of solar masses worth of new stars each year, depending on the gas content of each galaxy and its redshift. Typical merger SFRs are less than 100 new solar masses per year. This is large compared to our Galaxy, which makes only a few (~2) new stars each year. Though stars almost never get close enough to actually collide in galaxy mergers, giant molecular clouds rapidly fall to the center of the galaxy where they collide with other molecular clouds. These collisions then induce condensations of these clouds into new stars. We can see this phenomenon in merging galaxies in the nearby universe. Yet, this process was more pronounced during the mergers that formed most elliptical galaxies we see today, which likely occurred 1-10 billion years ago, when there was much more gas (and thus more molecular clouds) in galaxies. Also, away from the center of the galaxy gas clouds will run into each other producing shocks which stimulate the formation of new stars in gas clouds. The result of all this violence is that galaxies tend to have little gas available to form new stars after they merge. Thus if a galaxy is involved in a major merger, and then a few billion years pass, the galaxy will have very few young stars (see Stellar evolution) left. This is what we see in today's elliptical galaxies, very little molecular gas and very few young stars. It is thought that this is because elliptical galaxies are the end products of major mergers which use up the majority of gas during the merger, and thus further star formation after the merger is quenched.
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. Such a library of galaxy merger simulations can be found on the GALMER website. A study led by Jennifer Lotz of the Space Telescope Science Institute in Baltimore, Maryland created computer simulations in order to better understand images taken by the Hubble Telescope. Lotz's team tried to account for a broad range of merger possibilities, from a pair of galaxies with equal masses joining together to an interaction between a giant galaxy and a puny one. The team also analyzed different orbits for the galaxies, possible collision impacts, and how galaxies were oriented to each other. In all, the group came up with 57 different merger scenarios and studied the mergers from 10 different viewing angles.
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.
One study found that large galaxies merged with each other on average once over the past 9 billion years. Small galaxies were coalescing with large galaxies more frequently. Note that the Milky Way and the Andromeda Galaxy are thought to collide in about 4.5 billion years. The merging of these galaxies would classify as major as they have similar sizes. The result would therefore be an elliptical galaxy.
Some of the galaxies that are in the process of merging or are believed to have formed by merging are:
Arp 299 (parts of it are also known as IC 694 and NGC 3690) is a pair of colliding galaxies approximately 134 million light-years away in the constellation Ursa Major. Both of the galaxies involved in the collision are barred irregular galaxies.
It is not completely clear which object is historically called IC 694. According to some sources, the small appendage more than an arcminute northwest of the main pair is actually IC 694, not the primary (eastern) companion.The interaction of the two galaxies in Arp 299 produced young powerful starburst regions similar to those seen in II Zw 96. Eight supernovae have been detected in Arp 299: SN 1992bu, SN 1993G, SN 1998T, SN 1999D were observed in NGC 3690 while SN 1990al, SN 2005U, SN2010O and SN2010P were observed in IC 694.Galaxy And Mass Assembly survey
The Galaxy And Mass Assembly (GAMA) survey is a project to exploit the latest generation of ground-based wide-field survey facilities to study cosmology and galaxy formation and evolution. GAMA will bring together data from a number of world class instruments:
The Anglo-Australian Telescope (AAT),
The VLT Survey Telescope (VST)
The Visible and Infrared Survey Telescope for Astronomy (VISTA)
The Australian Square Kilometre Array Pathfinder (ASKAP)
The Herschel Space Observatory
The Galaxy Evolution Explorer (GALEX)Data from these instruments will be used to construct a state-of-the-art multi-wavelength database of ~375,000 galaxies in the local Universe over a 360 deg2 region of sky,
based on a spectroscopic redshift survey on the AAT's AAOmega spectrograph.
The main objective of GAMA is to study structure on scales of 1 kpc to 1 Mpc. This includes galaxy clusters, groups, mergers and coarse measurements of galaxy structure (i.e., bulges and discs). It is on these scales where baryons play a critical role in the galaxy formation and subsequent evolutionary processes and where our understanding of structure in the Universe breaks down.
GAMA's primary goal is to test the CDM paradigm of structure formation. In particular, the key scientific objectives are:
A measurement of the dark matter halo mass function of groups and clusters using group velocity dispersion measurements.
A comprehensive determination of the galaxy stellar mass function to Magellanic Cloud masses to constrain baryonic feedback processes.
A direct measurement of the recent galaxy merger rates as a function of mass, mass ratio, local environment and galaxy type.In August 2012 GAMA received worldwide attention for its announcement of a galaxy system very similar to our own Milky-Way Magellanic Cloud system, centred on GAMA202627.HXMM01
HXMM01, known more formally as 1HERMES S250 J022016.5−060143, is a starburst galaxy located in the northwestern portion of the constellation Cetus. Discovered in 2013 by a team at the University of California, Irvine, it was discovered that HXMM01 is actually still forming from its two parent galaxies as part of the "brightest, most luminous and most gas-rich submillimeter-bright galaxy merger known." When the merger is complete, HXMM01 will rapidly evolve to become a giant elliptical galaxy with a mass about four times that of the Milky Way. As of 2013, HXMM01 has been observed to form about 2,000 M☉ of stars every year, with an efficiency ten times greater than that of typical galaxies and far more than the Milky Way's 0.68–1.45 M☉ per year.Hypercompact stellar system
A hypercompact stellar system (HCSS) is a dense cluster of stars around a supermassive black hole that has been ejected from the center of its host galaxy. Stars that are close to the black hole at the time of the ejection will remain bound to the black hole after it leaves the galaxy, forming the HCSS.
The term "hypercompact" refers to the fact HCSSs are small in size compared with ordinary star clusters of similar luminosity. This is because the gravitational force from the supermassive black hole keeps the stars moving in very tight orbits about the center of the cluster.
The luminous X-ray source SDSS 1113 near the galaxy Markarian 177 would be the first candidate for an HCSS. Finding an HCSS would confirm the theory of gravitational wave recoil, and would prove that supermassive black holes can exist outside galaxies.Interacting galaxy
Interacting galaxies (colliding galaxies) are galaxies whose gravitational fields result in a disturbance of one another. An example of a minor interaction is a satellite galaxy's disturbing the primary galaxy's spiral arms. An example of a major interaction is a galactic collision, which may lead to a galaxy merger.LBG-2377
LBG-2377 is the most distant galaxy merger discovered, as of 2008, at a distance of 11.4 billion light years. This galaxy merger is so distant that the universe was in its infancy when its light was emitted. It is expected that this galaxy proto-cluster will merge to form a brightest cluster galaxy, and become the core of a larger galaxy cluster.NGC 2782
NGC 2782 is a peculiar spiral galaxy that formed after a galaxy merger in the constellation Lynx. The galaxy lies 75 million light years away from Earth, which means, given its apparent dimensions, that NGC 2782 is approximately 100,000 light years across. NGC 2782 has an active galactic nucleus and it is a starburst and a type 1 Seyfert galaxy. NGC 2782 is mentioned in the Atlas of Peculiar Galaxies in the category galaxies with adjacent loops.NGC 3921
NGC 3921 is a interacting galaxy in the northern constellation of Ursa Major. Estimates using redshift put the galaxy at about 59 million light years (18 megaparsecs) away. It was discovered on 14 April 1789 by William Herschel, and it was described as "pretty faint, small, round" by John Louis Emil Dreyer, the compiler of the New General Catalogue.NGC 3921 is the remnant of a galaxy merger. The two progenitor galaxies are thought to have been disk galaxies that collided about 700 million years ago. The image shows noticeable star formation and structures like loops, indicative of galaxies interacting. Because of this, NGC 3921 was included in Halton Arp's Atlas of Peculiar Galaxies under the designation Arp 224.Being a starburst galaxy, NGC 3921 has important features. One of them is an ultraluminous X-ray source, designated X-2, with an X-ray luminosity of 8×1039 erg/s. Additionally, two candidates globular clusters have been detected within NGC 3921. They are both fairly young, and are about half as massive as Omega Centauri: this demonstrates that mergers of gas-rich galaxies can also create more metal-rich globular clusters.NGC 3941
NGC 3941 is a barred lenticular galaxy located in the constellation Ursa Major. It is located at a distance of circa 40 million light years from Earth, which, given its apparent dimensions, means that NGC 3941 is about 40,000 light years across. It was discovered by William Herschel in 1787.NGC 4194
NGC 4194, the Medusa merger, is a pair of interacting galaxies in the constellation Ursa Major.
A region of extreme star formation 500 ly (150 pc) across exists in the center of the Eye of Medusa, the central gas-rich region.NGC 428
NGC 428 is a barred spiral galaxy in the constellation of Cetus (The Sea Monster), with its spiral structure distorted and warped, possibly the result of the collision of two galaxies. There appears to be a substantial amount of star formation occurring within NGC 428 and lacks well defined arms — a telltale sign of a galaxy merger. In 2015 the Hubble Space Telescope made a close-up shot of the galaxy with its Advanced Camera for Surveys and its Wide Field and Planetary Camera 2. The structure of NGC 428 has been compared to NGC 5645.NGC 4494
NGC 4494 is an elliptical galaxy located in the constellation Coma Berenices. It is located at a distance of circa 45 million light years from Earth, which, given its apparent dimensions, means that NGC 4494 is about 60,000 light years across. It was discovered by William Herschel in 1785.NGC 6240
NGC 6240 is a nearby ultraluminous infrared galaxy (ULIRG) in the constellation Ophiuchus. The galaxy is the remnant of a merger between two smaller galaxies. The collision between the two progenitors has resulted in a single larger galaxy with two distinct nuclei and a highly disturbed structure, including faint extensions and loops.NGC 720
NGC 720 is an elliptical galaxy located in the constellation Cetus. It is located at a distance of circa 80 million light years from Earth, which, given its apparent dimensions, means that NGC 720 is about 110,000 light years across. It was discovered by William Herschel on October 3, 1785. The galaxy is included in the Herschel 400 Catalogue. It lies about three and a half degrees south and slightly east from zeta Ceti.NGC 7252
NGC 7252 is a peculiar galaxy resulting from an interaction between two galaxies that started a billion years ago. It is located 220 million light years away in the constellation Aquarius. It is also called Atoms for Peace galaxy, a nickname which comes from its loop-like structure, made of stars, that resembles a diagram of an electron orbiting an atomic nucleus.NGC 7727
NGC 7727 is a peculiar galaxy in the constellation Aquarius.NGC 985
NGC 985 is a ring galaxy in the constellation of Cetus. It is located about 550 million light years away from Earth, which means, given its apparent dimensions, that NGC 985 is approximately 160,000 light years across. It was discovered by Francis Leavenworth in 1886. It is a type 1 Seyfert galaxy.NGC 985 is characterised by its ring shape. It is believed it was formed as a result of a galaxy merger. Further evidence supporting this theory is the observation of a second nucleus in NGC 985. When observed in infrared light, a second nucleus was found 3.8 arcseconds northwest of the active nucleus. It is much redder than the rest of the galaxy, indicating the presence of old stars. It has been suggested that the collision between a disk galaxy with another galaxy caused the formation of the ring and displaced the nucleus of the galaxy, creating an empty ring. Based on the kinematics of the galaxy, the secondary nucleus belonged to the intruder galaxy, while the active nucleus is associated with the main stellar component.As is common with merger remnants, NGC 985 has increased star formation rate, and as a result shines bright in the infrared. The total infrared luminosity of NGC 985 is 1.8×1011 L☉ and it is characterised as a luminous infrared galaxy. The total molecular gas mass of the galaxy is estimated to be 2×1010 M☉. Very large molecular clouds exist near the nuclei. They may be clouds gathering around the nucleus in the process of forming a disk around the two nuclei or molecular clouds disrupted by an outflow from the nucleus of the galaxy.NGC 985 is a powerful X-ray source, detected by ROSAT. It is a complex X-ray source, whose spectrum cannot be accounted for by a simple power law at 0.6 keV and suggests the presence of a warm absorber. The hard X-ray emission on the other hand is characterised by a simple power law. The X-ray flux, especially soft X-rays, diminished in NGC 985 in 2013. The variability of the X-ray and ultraviolet emission from the nucleus was observed using the XMM-Newton and Hubble Space Telescope respectively. These observations revealed the presence of outflowing wind from an accretion disk formed around a supermassive black hole that obstructed the nucleus in soft X-rays and UV. The nucleus is otherwise seen unobstructed.PKS 1302-102
PKS 1302-102 is a quasar in the Virgo constellation, located at a distance of approximately 1.1 Gpc (around 3.5 billion light-years). It has an apparent magnitude of about 14.9 mag in the V band with a redshift of 0.2784. The quasar is hosted by a bright elliptical galaxy, with two neighboring companions at distances of 3 kpc and 6 kpc. The light curve of PKS 1302-102 appears to be sinusoidal with an amplitude of 0.14 mag and a period of 1,884 ± 88 days, which suggests evidence of a supermassive black hole binary.X-shaped radio galaxy
X-shaped (or "winged") radio galaxies are a class of extragalactic radio source that exhibit two, low-surface-brightness radio lobes (the "wings") oriented at an angle to the active, or high-surface-brightness, lobes. Both sets of lobes pass symmetrically through the center of the elliptical galaxy that is the source of the lobes, giving the radio galaxy an X-shaped morphology as seen on radio maps (see figure).
X-shaped sources were first described by J. P. Leahy and P. Parma in 1992, who presented a list of 11 such objects. The X-shaped galaxies have received much attention following the suggestion in 2002 that they might be the sites of spin-flips associated with the recent coalescence of two supermassive black holes.