{{Short description|Galaxies with interacting gravitational fields}} {{More citations needed|date=May 2009}} [[File:NGC 3169 NGC 3166.jpg|thumb|250px|NGC 3169 (left) and NGC 3166 (right), located in the constellation Sextans display some curious features, showing that both galaxies are close enough to feel the distorting gravitational influence of the other. Image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at the La Silla Observatory.]]

'''Interacting galaxies''', also known as '''''colliding galaxies''''', are two or more galaxies whose gravitational fields result in a disturbance of one another. There are several types of galactic interactions, including major interactions, minor interactions, and galaxy harassment. Major interactions occur between galaxies with similar amounts of mass, whereas minor interactions involve galaxies with masses that vary significantly.<ref name=":0" /> An example of a minor interaction is a satellite galaxy disturbing the primary galaxy's spiral arms. An example of a major interaction is a galactic collision, such as the one that astronomers estimate will happen in the future between the Milky Way and Andromeda.<ref name=":2" /> Collisions may lead to galaxy mergers and may also lead to other phenomena such as star formation<ref>{{Cite journal |last1=Zee |first1=Woong-Bae G. |last2=Yoon |first2=Suk-Jin |date=2025-12-01 |title=Living with Neighbors. VI. Unraveling the Dual Impact of Bars on Star Formation in Paired Galaxies Using DESI |journal=The Astrophysical Journal |volume=994 |issue=2 |pages=269 |doi=10.3847/1538-4357/ae160a |doi-access=free|arxiv=2510.18937 |bibcode=2025ApJ...994..269Z |issn=0004-637X}}</ref> and black hole activity<ref name=":3">{{Cite journal |last1=Goulding |first1=A. D. |last2=Forman |first2=W. R. |last3=Hickox |first3=R. C. |last4=Jones |first4=C. |last5=Murray |first5=S. S. |last6=Paggi |first6=A. |last7=Ashby |first7=M. L. N. |last8=Coil |first8=A. L. |last9=Cooper |first9=M. C. |last10=Huang |first10=J.-S. |last11=Kraft |first11=R. |last12=Newman |first12=J. A. |last13=Weiner |first13=B. J. |last14=Willner |first14=S. P. |date=2014-02-11 |title=TRACING THE EVOLUTION OF ACTIVE GALACTIC NUCLEI HOST GALAXIES OVER THE LAST 9 Gyr OF COSMIC TIME |url=https://iopscience.iop.org/article/10.1088/0004-637X/783/1/40 |journal=The Astrophysical Journal |volume=783 |issue=1 |pages=40 |doi=10.1088/0004-637X/783/1/40 |arxiv=1310.8298 |bibcode=2014ApJ...783...40G |issn=0004-637X}}</ref>.

==Satellite interaction== A giant galaxy interacting with its satellites is common. A satellite's gravity could attract one of the primary's spiral arms. Alternatively, the secondary satellite can dive into the primary galaxy, as in the Sagittarius Dwarf Elliptical Galaxy diving into the Milky Way. 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 name=":0">{{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> thumb|Animation of Galaxy Collision

==Galaxy collision== thumb|Gravitationally lensed galactic merger H-ATLAS J142935.3-002836.<ref name=":1">{{cite news|title=Best View Yet of Merging Galaxies in Distant Universe|url=https://www.eso.org/public/news/eso1426/|access-date=26 August 2014|work=ESO Press Release}}</ref>

Colliding galaxies are common during galaxy evolution.<ref>{{Cite web|url=https://www.space.com/29157-hubble-space-telescope-science-legacy.html|title=How the Hubble Space Telescope Changed Our View of the Cosmos|author1=Nola Taylor Tillman|date=April 21, 2015|website=Space.com}}</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. As a result, some galaxy features such as star formation, shape, and size are impacted by interactions with other galaxies.<ref name=":1" /> [[File:Hubble Views a Cosmic Interaction - Flickr - NASA Hubble.jpg|thumb|This image, captured by the NASA/ESA Hubble Space Telescope, shows two interacting galaxies, NGC 169 (bottom) and IC 1559 (top).]] Colliding may lead to merging if two galaxies collide and do not have enough momentum to continue traveling after the collision. As with other galaxy collisions, the merging of two galaxies may create a starburst region of new stars.<ref>{{Cite web |last=Gianopoulos |first=Andrea |date=2022-02-18 |title=Galaxy Collision Creates 'Space Triangle' in New Hubble Image |url=http://www.nasa.gov/image-feature/goddard/2022/galaxy-collision-creates-space-triangle-in-new-hubble-image |access-date=2022-12-01 |website=NASA}}</ref> This starburst is an example of an increased star formation rate within the merged galaxies. A study conducted in 2024 shows that a key component in determining this change in star formation rate is the masses of the two galaxies involved. It suggests that if the two galaxies are of similar mass to begin with, this increase in star formation rate will be greater.<ref>Robin, T; Kartha, Sreeja S; Akhil Krishna, R; Krishnan, Ujjwal; Mathew, Blesson; Cysil, T B; Patra, Narendra Nath; Shridharan, B (2024-11-01). "[https://academic.oup.com/mnras/article/534/3/1902/7774402 The Interaction Jigsaw: investigating star formation in interacting galaxies]". ''Monthly Notices of the Royal Astronomical Society''. '''534''' (3): 1902–1912. doi:10.1093/mnras/stae2211. ISSN 0035-8711.</ref> When a starburst region is created, the galaxies 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 of 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">{{cite web|title=GALMER |url=http://galmer.obspm.fr |access-date=27 March 2010}}</ref>

== Galaxy harassment == {{Main|Galaxy harassment}} 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 due to the high galactic density.

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. The result would be the conversion of (late type) low-luminosity spiral galaxies into dwarf spheroidals and dwarf ellipticals.<ref>{{Cite web|url=https://supernova.lbl.gov/~evlinder/umass/sumold/nk/harass.html|title=Galaxy Harassment|website=supernova.lbl.gov}}</ref>

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 disc systems transformed by the above-mentioned interactions.<ref>{{Cite journal |last1=Barazza |first1=F. D. |last2=Binggeli |first2=B. |last3=Jerjen |first3=H. |date=September 2002 |title=More evidence for hidden spiral and bar features in bright early-type dwarf galaxies |journal=Astronomy & Astrophysics |volume=391 |issue=3 |pages=823–831 |arxiv=astro-ph/0206275 |bibcode=2002A&A...391..823B |doi=10.1051/0004-6361:20020875 |issn=0004-6361 |s2cid=844270}}</ref> The existence of similar structures in isolated early-type dwarf galaxies, such as LEDA 2108986, has undermined this hypothesis.<ref>{{Cite journal |last1=Graham |first1=Alister W. |last2=Janz |first2=Joachim |last3=Penny |first3=Samantha J. |last4=Chilingarian |first4=Igor V. |last5=Ciambur |first5=Bogdan C. |last6=Forbes |first6=Duncan A. |last7=Davies |first7=Roger L. |date=May 2017 |title=Implications for the Origin of Early-type Dwarf Galaxies: A Detailed Look at the Isolated Rotating Early-type Dwarf Galaxy LEDA 2108986 (CG 611), Ramifications for the Fundamental Plane's {S}_{K}^{2} Kinematic Scaling, and the Spin-Ellipticity Diagram |journal=The Astrophysical Journal |volume=840 |issue=2 |pages=68 |arxiv=1705.03587 |bibcode=2017ApJ...840...68G |doi=10.3847/1538-4357/aa6e56 |issn=0004-637X |doi-access=free}}</ref><ref>{{Cite journal |last1=Janz |first1=Joachim |last2=Penny |first2=Samantha J. |last3=Graham |first3=Alister W. |last4=Forbes |first4=Duncan A. |last5=Davies |first5=Roger L. |date=July 2017 |title=Implications for the origin of early-type dwarf galaxies – the discovery of rotation in isolated, low-mass early-type galaxies |journal=Monthly Notices of the Royal Astronomical Society |volume=468 |issue=3 |pages=2850–2864 |arxiv=1703.04975 |bibcode=2017MNRAS.468.2850J |doi=10.1093/mnras/stx634 |issn=0035-8711 |doi-access=free}}</ref>

==Notable examples== thumb|250px|Montage of some well known interacting galaxies {| class="wikitable sortable" !Name !Type !Distance<br>(million ly) !Magnitude !Notes |- |Milky Way Galaxy, LMC and 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 |- |Butterfly Galaxies NGC 4567/8 | SA(rs)bc / SA(rs)bc |style="text-align:right;"| 60 |style="text-align:right;"| +10.9 |Early phase of interaction |- |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 |- |NGC 2936 |Irr |style="text-align:right;"| 352 |style="text-align:right;"|+12.9 |? |}

==Andromeda–Milky Way collision== {{main|Andromeda–Milky Way collision}}

Astronomers have estimated the Milky Way Galaxy will collide with the Andromeda Galaxy in about 4.5 billion years. Thanks to the Hubble Space Telescope, astronomers were able to more accurately track Andromeda's movement, leading to the conclusion that the two galaxies will temporarily touch before eventually undergoing an ultimate merging.<ref name=":2">{{Cite journal |last=Cowen |first=Ron |date=2012-05-31 |title=Andromeda on collision course with the Milky Way |url=https://www.nature.com/articles/nature.2012.10765 |journal=Nature |language=en |doi=10.1038/nature.2012.10765 |issn=1476-4687|url-access=subscription }}</ref> Some think the two spiral galaxies will eventually merge to become an elliptical galaxy whose gravitational interactions will fling various celestial bodies outward, evicting them from the resulting elliptical galaxy.<ref name="muir">{{Cite web |last=Hazel |first=Muir |date=14 May 2007 |title=Galactic merger to 'evict' Sun and Earth |url=https://www.newscientist.com/article/dn11852-galactic-merger-to-evict-sun-and-earth.html |url-status=live |archive-url=https://web.archive.org/web/20140420022909/http://www.newscientist.com/article/dn11852-galactic-merger-to-evict-sun-and-earth.html#.VDVeCfl_uVA |archive-date=20 April 2014 |access-date=2014-10-07 |publisher=New Scientist }}</ref><ref>{{Cite magazine |last1=Loeb |first1=Abraham |author-link1=Avi Loeb |last2=Cox |first2=T. J. |date=June 2008 |title=Our galaxy's collision with Andromeda |magazine=Astronomy |page=28 }}</ref> or perhaps a large disc 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=1 | bibcode= 2014ApJS..214....1U | doi=10.1088/0067-0049/214/1/1|arxiv = 1407.6873 | year=2014 | s2cid=716993 }}</ref>

== Black holes and interacting galaxies == According to recent studies, galactic interactions may play an important role in triggering black hole activity by driving gas toward the central regions of galaxies, where it fuels supermassive black hole accretion and AGN activity.<ref>{{Cite journal |last=Diriba Gonfa Tolasa |date=2025-04-30 |title=The Interplay Between Black Holes and Galaxy Formation: A Cosmological Perspective |url=https://ijarise.org/index.php/ijarise/article/view/109 |journal=International Journal of Advanced Research and Interdisciplinary Scientific Endeavours |volume=2 |issue=4 |pages=597–612 |doi=10.61359/11.2206-2522 |issn=3048-7021|doi-access=free }}</ref> Both observational evidence and simulations suggest that this inflow of gas can ignite active galactic nucleus (AGN) activity. A 2014 study<ref name=":3" /> suggests that AGN fueling depends on the gas content and evolutionary state of the host galaxy. They found that blue, star forming galaxies host radiatively efficient AGN fueled by cold gas, while passive and more massive galaxies host radio AGN fueled by mechanically dominated accretion. A 2020 study<ref>{{Cite journal |last1=Greene |first1=Jenny E. |last2=Setton |first2=David |last3=Bezanson |first3=Rachel |last4=Suess |first4=Katherine A. |last5=Kriek |first5=Mariska |last6=Spilker |first6=Justin S. |last7=Feldmann |first7=Robert |last8=Goulding |first8=Andy D. |date=2020 |title=The Role of Active Galactic Nuclei in the Quenching of Massive Galaxies in the SQuiGGLE Survey |journal=The Astrophysical Journal Letters |volume=899 |pages=L9 |doi=10.3847/2041-8213/aba534 |doi-access=free |arxiv=2007.02967}}</ref> found that the incidence of AGN is significantly higher in quenched (post-starburst) galaxies. They argue that this trend most likely reflects a continued availability of residual gas, rather than quenching being a direct consequence of black hole feedback on the host galaxy. Together, these studies support an evolutionary sequence in which galaxy interactions, star formation, and AGN activity are closely intertwined in shaping galaxy evolution.

==See also== * NGC 7318

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

==External links== {{Commons category|Interacting galaxies}} *[https://iowastatedaily.com/66625/news/isu-researchers-study-galaxy-collisions/ Galaxy Collisions] *[https://web.archive.org/web/20070220192943/http://www.iac.es/gabinete/noticias/2001/mar08i.htm Galactic cannibalism] *[http://galmer.obspm.fr GALMER: Galaxy Merger Simulations]

{{Galaxy|state=collapsed}}

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