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{{Refimprove|date=May 2009}}
[[File:NGC 3169 NGC 3166.jpg|thumb|250px|[[NGC 3169]] (left) and [[NGC 3166]] (right) display some curious features, demonstrating that each member of the duo is close enough to feel the distorting gravitational influence of the other. Image from the Wide Field Imager on the MPG/[[ESO]] [[MPG/ESO telescope|2.2-metre telescope]] at the [[La Silla Observatory]].]]

'''Interacting galaxies''' ('''''colliding galaxies''''') are [[galaxy|galaxies]] whose [[gravity|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 arm]]s. An example of a major interaction is a galactic collision, which may lead to a [[galaxy merger]].

==Satellite interaction==
A giant galaxy interacting with its [[satellite galaxy|satellites]] is common. A satellite's gravity could attract one of the primary's [[spiral arm]]s, 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 Spheroidal Galaxy|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.<ref>{{Cite web|url=http://hubblesite.org/news_release/news/2008-02|title=HubbleSite: News - Hubble Finds that "Blue Blobs" in Space Are Orphaned Clusters of Stars|website=hubblesite.org|access-date=2017-05-24}}</ref>
[[File:Galaxy Collision Animation- James Webb Space Telescope Science.webm|thumb|Animation of Galaxy Collision]]

==Galaxy collision==
[[File:Merging galaxies in the distant Universe through a gravitational magnifying glass.jpg|thumb|Merging galaxies in the distant Universe through a gravitational magnifying glass.<ref>{{cite news|title=Best View Yet of Merging Galaxies in Distant Universe|url=http://www.eso.org/public/news/eso1426/|accessdate=26 August 2014|work=ESO Press Release}}</ref>]]

Colliding galaxies are common during [[galaxy formation and evolution|galaxy evolution]].<ref>[http://www.space.com/29157-hubble-space-telescope-science-legacy.html space.com 2015-04-21 How the Hubble Space Telescope Changed Our View of the Cosmos-Galactic Collisions Photographs]</ref> 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 [[galaxy merger|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
<ref name= "galmer.obspm.fr">GALMER [http://galmer.obspm.fr 27 March 2010]</ref>

==Gallery==

<gallery>
Image:Close encounter IRAS 06076-2139.jpg|Close encounter at [[IRAS 06076-2139]].<ref>{{cite web|title=Close encounter|url=https://www.spacetelescope.org/images/potw1719a/|website=www.spacetelescope.org|accessdate=8 May 2017}}</ref>
Image:Heic1709a.jpg|Interacting galaxies [[NGC 4302]] and [[NGC 4298]] both located 55 million light-years away.<ref>{{cite web|title=A close galactic pair|url=https://www.spacetelescope.org/images/heic1709a/|website=www.spacetelescope.org|accessdate=21 April 2017}}</ref>
Image:NGC 6052.jpg|Galaxy [[NGC 6052]] merging into a single structure.<ref>{{cite web|title=Two become one|url=http://www.spacetelescope.org/images/potw1552a/|accessdate=28 December 2015}}</ref>
Image:Galactic soup.jpg|Galaxy pair Zw I 136.<ref>{{cite news|title=Galactic soup|url=http://www.spacetelescope.org/images/potw1432a/|accessdate=18 August 2014|work=ESA/Hubble Picture of the Week}}</ref>
Image:The messy result of a galactic collision.jpg|[[ESO 576-69]] is believed to be the nucleus of a former spiral galaxy.<ref>{{cite news|title=The messy result of a galactic collision|url=http://www.spacetelescope.org/images/potw1321a/|accessdate=29 May 2013|newspaper=ESA/Hubble Picture of the Week}}</ref>
Image:Whirlpool (M51).jpg|The [[Whirlpool Galaxy]] with its [[satellite galaxy|satellite]] [[Irregular Galaxy NGC 5195|NGC 5195]].
Image:NGC4676.jpg|The [[Mice Galaxies]].
File:Colliding Galaxies Create Active Galactic Nuclei.OGG|This simulation follows the collision of two spiral galaxies that harbour giant black holes.
File:Defying cosmic convention.jpg|[[NGC 3447]] comprises a couple of interacting galaxies.<ref>{{cite web|title=Defying cosmic convention|url=https://www.spacetelescope.org/images/potw1712a/|website=www.spacetelescope.org|accessdate=20 March 2017}}</ref>
Image:NGC 1510 and NGC 1512 in UV Light.jpg|[[Barred spiral galaxy]] [[NGC 1512]] in the process of a lengthy merger with [[dwarf galaxy|dwarf]] [[elliptical galaxy]] [[NGC 1510]].
</gallery>

==Galactic cannibalism==
[[File:The last waltz.jpg|thumb|2MASX J16270254+4328340 galaxy has merged with another galaxy leaving a fine mist, made of millions of stars, spewing from it in long trails.<ref>{{cite web|title=The last waltz|url=http://www.spacetelescope.org/images/potw1547a/|accessdate=14 December 2015}}</ref>]]
'''Galactic cannibalism''' refers to the process in which a large [[galaxy]], through [[galactic tide|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 galaxy|elliptical galaxies]] may also result.

It has been suggested that galactic cannibalism is currently occurring between the [[Milky Way]] and the [[Magellanic clouds|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==
Galaxy harassment is a type of interaction between a low-luminosity galaxy and a brighter one that takes place within rich [[galaxy cluster]]s, such as [[Virgo Cluster|Virgo]] and [[Coma Cluster|Coma]], where galaxies are moving at high [[relative speed]]s and suffering frequent encounters with other systems of the cluster by the high galactic density of the latter.
According to [[computer simulation]]s, the interactions convert the affected systems into disturbed [[barred spiral galaxy|barred spiral galaxies]] and produces [[starburst galaxy|starbursts]] on them followed, if more encounters occur, by loss of [[angular momentum]] and heating of their gas.

The result would be the conversion of (late type) low-luminosity spiral galaxies into [[dwarf spheroidal galaxy|dwarf spheroidals]] and [[dwarf elliptical galaxy|dwarf ellipticals]].<ref>[http://supernova.lbl.gov/~evlinder/umass/sumold/nk/harass.html Galaxy Harassment]</ref>

Evidence for the hypothesis has been found by studying early-type dwarf galaxies of the Virgo Cluster and finding structures, such as disks and spiral arms. That suggests they are former [[disk galaxy|disk systems]], transformed by the mentioned interactions.<ref>[http://adsabs.harvard.edu/abs/2002A&A...391..823B More evidence for hidden spiral and bar features in bright early-type dwarf galaxies]</ref>

==Notable interacting galaxies==
{| class="wikitable" style="border-collapse:collapse;" cellpadding="2"
|-
!style="background:#efefef;"|Name
!style="background:#efefef;"|[[Galaxy morphological classification|Type]]
!style="background:#efefef;"|Distance<br>(million [[light-years|ly]])
!style="background:#efefef;"|[[Apparent magnitude|Magnitude]]
!style="background:#efefef;"|Notes
|-
|[[Milky Way Galaxy]], [[Large Magellanic Cloud|LMC]] and [[Small Magellanic Cloud|SMC]]
|SBc/SB(s)m/SB(s)m pec
|style="text-align:right;"|0
|style="text-align:right;"|
|Satellites interacting with their primary
|-
|[[Whirlpool Galaxy]] (M51)
|SAc (SB0-a)
|style="text-align:right;"|37
|style="text-align:right;"|+8.4
|Satellite interacting with its primary
|-
|[[NGC 1097]]
|SB(s)bc (E6)
|style="text-align:right;"|45
|style="text-align:right;"|+9.5
|Satellite interacting with its primary
|-
|[[NGC 2207 and IC 2163]]
|SAc/SAbc
|style="text-align:right;"|114
|style="text-align:right;"|+11
|galaxies going through the '''first phase''' in galactic collision
|-
|[[Mice Galaxies]] (NGC 4676A and NGC 4676B)
|S0/SB(s)ab
|style="text-align:right;"|300
|style="text-align:right;"|+13.5
|galaxies going through the '''second phase''' in galactic collision
|-
|[[Antennae Galaxies]] (NGC 4038/9)
|SAc/SBm
|style="text-align:right;"|45
|style="text-align:right;"|+10.3
|galaxies going through the '''third phase''' in galactic collision
|-
|[[NGC 520]]
|S
|style="text-align:right;"|100
|style="text-align:right;"|+11.3
|galaxies going through the '''third phase''' in galactic collision
|}

==Future collision of the Milky Way with Andromeda==
{{main|Andromeda–Milky Way 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 galaxy|spiral galaxies]] will eventually merge to become an [[elliptical galaxy]]<ref name = "muir">whose gravitational interactions will fling various celestial bodies outward, evicting them from the resulting elliptical galaxy.{{cite web

| author = Hazel Muir
| publisher = [[New Scientist]]
| date = 2007-05-14
| title = Galactic merger to 'evict' Sun and Earth
| url = https://www.newscientist.com/article/dn11852-galactic-merger-to-evict-sun-and-earth.html#.VDVdDvl_uVB
| accessdate = 2014-10-07
| archiveurl = https://web.archive.org/web/20140420022909/http://www.newscientist.com/article/dn11852-galactic-merger-to-evict-sun-and-earth.html#.VDVeCfl_uVA
| archivedate = 20 April 2014
| deadurl = no
}}</ref><ref>[[Astronomy (magazine)|Astronomy]], June 2008, page 28, by [[Abraham Loeb]] and [[T.J.Cox]]</ref> or perhaps a large [[disk galaxy]].<ref name="Ueda2014">{{cite journal
| author=Junko Ueda
| display-authors=etal
| title=Cold molecular gas in merger remnants. I. Formation of molecular gas disks
| journal=The Astrophysical Journal Supplement Series
| volume=214
| issue=1
| pages=
| bibcode= 2014ApJS..214....1U
| doi=10.1088/0067-0049/214/1/1|arxiv = 1407.6873 }}</ref>

==See also==
* [[NGC 7318]]

==References==
{{Reflist|2}}

==External links==
{{Commons category|Interacting galaxies}}
*[http://www.public.iastate.edu/~curt/cg/homepage.html Galaxy Collisions]
*[https://web.archive.org/web/20070220192943/http://www.iac.es/gabinete/noticias/2001/mar08i.htm Galactic cannibalism]
*[http://burro.cwru.edu/JavaLab/GalCrashWeb/ Galactic Collision Simulation]
*[http://galmer.obspm.fr GALMER: Galaxy Merger Simulations]

{{Galaxy}}

{{DEFAULTSORT:Interacting Galaxy}}
[[Category:Interacting galaxies| ]]
[[Category:Extragalactic astronomy]]
[[Category:Articles containing video clips]]

Interacting galaxy - Revision history

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