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.
A giant galaxy interacting with its satellites is common. A satellite's gravity could attract one of the primary's spiral arms, or the secondary satellite's path could coincide with the position of the primary satellite's and so would dive into the primary galaxy (the Sagittarius Dwarf Elliptical Galaxy into the Milky Way being an example of the latter). That can possibly trigger a small amount of star formation. Such orphaned clusters of stars were sometimes referred to as "blue blobs" before they were recognized as stars.
Colliding galaxies are common during galaxy evolution. The extremely tenuous distribution of matter in galaxies means these are not collisions in the traditional sense of the word, but rather gravitational interactions.
Colliding may lead to merging if two galaxies collide and do not have enough momentum to continue traveling after the collision. In that case, they fall back into each other and eventually merge into one galaxy after many passes through each other. If one of the colliding galaxies is much larger than the other, it will remain largely intact after the merger. The larger galaxy will look much the same, while the smaller galaxy will be stripped apart and become part of the larger galaxy. When galaxies pass through each other, unlike during mergers, they largely retain their material and shape after the pass.
Galactic collisions are now frequently simulated on computers, which use realistic physics principles, including the simulation of gravitational forces, gas dissipation phenomena, star formation, and feedback. Dynamical friction slows the relative motion galaxy pairs, which may possibly merge at some point, according to the initial relative energy of the orbits. A library of simulated galaxy collisions can be found at the Paris Observatory website: GALMER 
Galactic cannibalism refers to the process in which a large galaxy, through tidal gravitational interactions with a companion, merges with that companion; that results in a larger, often irregular galaxy.
The most common result of the gravitational merger between two or more galaxies is an irregular galaxy, but elliptical galaxies may also result.
It has been suggested that galactic cannibalism is currently occurring between the Milky Way and the Large and Small Magellanic Clouds. Streams of gravitationally-attracted hydrogen arcing from these dwarf galaxies to the Milky Way is taken as evidence for the theory.
Galaxy harassment is a type of interaction between a low-luminosity galaxy and a brighter one that takes place within rich galaxy clusters, such as Virgo and Coma, where galaxies are moving at high relative speeds and suffering frequent encounters with other systems of the cluster by the high galactic density of the latter. According to computer simulations, the interactions convert the affected galaxy disks into disturbed barred spiral galaxies and produces starbursts followed by, if more encounters occur, loss of angular momentum and heating of their gas.
Evidence for the hypothesis had been claimed by studying early-type dwarf galaxies in the Virgo Cluster and finding structures, such as disks and spiral arms, which suggest they are former disk systems transformed by the above-mentioned interactions. However, the existence of similar structures in isolated early-type dwarf galaxies, such as LEDA 2108986, has undermined this hypothesis
|Milky Way Galaxy, LMC and SMC||SBc/SB(s)m/SB(s)m pec||0||Satellites interacting with their primary|
|Whirlpool Galaxy (M51)||SAc (SB0-a)||37||+8.4||Satellite interacting with its primary|
|NGC 1097||SB(s)bc (E6)||45||+9.5||Satellite interacting with its primary|
|NGC 2207 and IC 2163||SAc/SAbc||114||+11||galaxies going through the first phase in galactic collision|
|Mice Galaxies (NGC 4676A and NGC 4676B)||S0/SB(s)ab||300||+13.5||galaxies going through the second phase in galactic collision|
|Antennae Galaxies (NGC 4038/9)||SAc/SBm||45||+10.3||galaxies going through the third phase in galactic collision|
|NGC 520||S||100||+11.3||galaxies going through the third phase in galactic collision|
Astronomers have estimated the Milky Way galaxy, will collide with the Andromeda galaxy in about 4.5 billion years. It is thought that the two spiral galaxies will eventually merge to become an elliptical galaxy or perhaps a large disk galaxy.
Arp 187 is a radio galaxy and merger remnant located in the constellation Eridanus. It is an interacting galaxy pair (MCG-02-13-040). It is included in the Atlas of Peculiar Galaxies in the category galaxies with narrow filaments.
The galaxy has two prominent radio lobes, however the emission of its AGN in X-ray is low, and so is the thermal output of the AGN torus as observed in infrared, suggesting that it has been quenched. The detection of a narrow line region (with dimensions up to 10 kpc) with still active emission mean that the AGN was still active up to at least 104-5 years ago. In the nucleus of Arp 187 is predicted to lie a supermassive black hole with estimated mass around 6.7 × 108 M☉.Atlas of Peculiar Galaxies
The Atlas of Peculiar Galaxies is a catalog of peculiar galaxies produced by Halton Arp in 1966. A total of 338 galaxies are presented in the atlas, which was originally published in 1966 by the California Institute of Technology. The primary goal of the catalog was to present photographs of examples of the different kinds of peculiar structures found among galaxies.Eyes Galaxies
The Eyes Galaxies (NGC 4435-NGC 4438, also known as Arp 120) are a pair of galaxies about 52 million light-years away in the constellation Virgo. The pair are members of the string of galaxies known as Markarian's Chain.IC 335
IC 335 is a lenticular galaxy about 60 million light years (18 million parsecs) away, in the constellation Fornax. It is part of the Fornax Cluster.IC 335 appears very similar to NGC 4452, a lenticular galaxy in Virgo. Both galaxies are edge-on, meaning that their characteristics, like spiral arms, are hidden. Lenticular galaxies like these are thought to be intermediate between spiral galaxies and elliptical galaxies, and like elliptical galaxies, they have very little gas for star formation. IC 335 may have once been a spiral galaxy that ran out of interstellar medium, or it may have collided with a galaxy in the past and thus used up all of its gas (See Interacting galaxy).List of NGC objects (4001–5000)
This is a list of NGC objects 4001–5000 from the New General Catalogue (NGC). The astronomical catalogue is composed mainly of star clusters, nebulae, and galaxies. Other objects in the catalogue can be found in the other subpages of the list of NGC objects.
The constellation information in these tables is taken from The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters by J. L. E. Dreyer, which was accessed using the "VizieR Service". Galaxy types are identified using the NASA/IPAC Extragalactic Database. The other data of these tables are from the SIMBAD Astronomical Database unless otherwise stated.List of NGC objects (5001–6000)
This is a list of NGC objects 5001–6000 from the New General Catalogue (NGC). The astronomical catalogue is composed mainly of star clusters, nebulae, and galaxies. Other objects in the catalogue can be found in the other subpages of the list of NGC objects.
The constellation information in these tables is taken from The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters by J. L. E. Dreyer, which was accessed using the "VizieR Service". Galaxy types are identified using the NASA/IPAC Extragalactic Database. The other data of these tables are from the SIMBAD Astronomical Database unless otherwise stated.List of NGC objects (6001–7000)
This is a list of NGC objects 6001–7000 from the New General Catalogue (NGC). The astronomical catalogue is composed mainly of star clusters, nebulae, and galaxies. Other objects in the catalogue can be found in the other subpages of the list of NGC objects.
The constellation information in these tables is taken from The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters by J. L. E. Dreyer, which was accessed using the "VizieR Service". Galaxy types are identified using the NASA/IPAC Extragalactic Database. The other data of these tables are from the SIMBAD Astronomical Database unless otherwise stated.NGC 1097
NGC 1097 is a barred spiral galaxy about 45 million light years away in the constellation Fornax. It was discovered by William Herschel on 9 October 1790. It is a severely interacting galaxy with obvious tidal debris and distortions caused by interaction with the companion galaxy NGC 1097A. Three supernovae (SN 1992bd, SN 1999eu, and SN 2003B) have been observed in NGC 1097 since 1992.NGC 1553
NGC 1553 is a prototypical lenticular galaxy in the constellation Dorado. It is the second brightest member of the Dorado Group of galaxies. British astronomer John Herschel discovered NGC 1553 on December 5, 1834 using an 18.7 inch reflector.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 5195
NGC 5195 (also known as Messier 51b or M51b) is a dwarf galaxy that is interacting with the Whirlpool Galaxy (also known as M51a or NGC 5194). Both galaxies are located approximately 25 million light-years away in the constellation Canes Venatici. Together, the two galaxies are one of the most famous interacting galaxy pairs.NGC 5713
NGC 5713 is a peculiar, asymmetric galaxy in the constellation Virgo. Although classified as a spiral galaxy by most galaxy catalogs, NGC 5713 galaxy is very different from most normal spiral galaxies. While most spiral galaxies either have either two well-defined spiral arms or a filamentary spiral-like structure, this spiral galaxy has only one visible spiral arm in its disk. This makes it a galaxy of the Magellanic type. Gravitational interactions with the nearby spiral galaxy NGC 5719 may be responsible for producing the disturbed, asymmetric structure including the single spiral arm.
NGC 5713 is at the center of a small group of spiral galaxies that also includes NGC 5691, NGC 5705, and NGC 5719.NGC 90
NGC 90 is an interacting spiral galaxy estimated to be about 300 million light-years away in the constellation of Andromeda. It was discovered by R. J. Mitchell in 1854 and its apparent magnitude is 13.7. The galaxy is currently interacting with NGC 93 and exhibits two highly elongated and distorted spiral arms with bright blue star clusters indicative of star formation, likely caused by the interaction with its neighbor.
NGC 90 and NGC 93 form the interacting galaxy pair Arp 65.NGC 93
NGC 93 is an interacting spiral galaxy estimated to be about 260 million light-years away in the constellation of Andromeda. It was discovered by R. J. Mitchell in 1854. The galaxy is currently interacting with NGC 90 and has some signs of interacting with it.
NGC 93 and NGC 90 form the interacting galaxy pair Arp 65.Peculiar galaxy
A peculiar galaxy is a galaxy of unusual size, shape, or composition. Between five and ten percent of known galaxies are categorized as peculiar. Astronomers have identified two types of peculiar galaxies: interacting galaxies and active galactic nuclei (AGN).When two galaxies come close to each other, their mutual gravitational forces can cause them to acquire highly irregular shapes. The terms 'peculiar galaxy' and 'interacting galaxy' have now become synonymous because the majority of peculiar galaxies attribute their forms to such gravitational forces.SN 1999ec
SN 1999ec was a type Ib supernova that was discovered in the interacting galaxy NGC 2207 on October 2, 1999. It was found on images taken with the Katzman Automatic Imaging Telescope at the Lick Observatory. The progenitor is estimated to have had 38 times the mass of the Sun and was 5.34 million years old at the time of the outburst.SN 2004gt
SN 2004GT was a type Ic supernova that happened in the interacting galaxy NGC 4038 on December 12, 2004. The event occurred in a region of condensed matter in the western spiral arm. The progenitor was not identified from older images of the galaxy, and is either a type WC Wolf-Rayet star with a mass over 40 times that of the Sun, or a star 20 to 40 times as massive as the Sun in a binary star system.Tidal tail
A tidal tail is a thin, elongated region of stars and interstellar gas that extends into space from a galaxy. Tidal tails occur as a result of galactic tide forces between interacting galaxies. Examples of galaxies with tidal tails include the Tadpole Galaxy and the Mice Galaxies. Tidal forces can eject a significant amount of a galaxy's gas into the tail; within the Antennae Galaxies, for example, nearly half of the observed gaseous matter is found within the tail structures. Within those galaxies which have tidal tails, approximately 10% of the galaxy's stellar formation takes place in the tail. Overall, roughly 1% of all stellar formation in the known universe occurs within tidal tails.Some interacting galaxy pairs have two distinct tails, as is the case for the Antennae Galaxies, while other systems have only one tail. Most tidal tails are slightly curved due to the rotation of the host galaxies. Those that are straight may actually be curved but still appear to be straight if they are being viewed edge-on.Void galaxy
Void galaxies are galaxies which exist in cosmological voids. Few galaxies exist in voids; most galaxies exist in sheets, walls and filaments that surround voids and supervoids. Many of the void galaxies are connected through void filaments or tendrils, lightweight versions of the regular galaxy filaments that surround voids, to each other. These filaments are often straighter than their regular counterparts due to the lack of influence by surrounding filaments. These filaments can even be rich enough to form poor groups of galaxies. The void galaxies themselves are thought to represent pristine examples of galactic evolution, having few neighbours, and likely to have formed from pure intergalactic gas.