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 disturbing the primary galaxy's spiral arms. An example of a major interaction is a galactic collision, which may lead to a galaxy merger.

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 2.2-metre telescope at the La Silla Observatory.

Satellite interaction

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.[1]

Galaxy collision
Merging galaxies in the distant Universe through a gravitational magnifying glass.[2]

Colliding galaxies are common during galaxy evolution.[3] 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 [4]

Galactic cannibalism
2MASX J16270254+4328340 galaxy has merged with another galaxy leaving a fine mist, made of millions of stars, spewing from it in long trails.[12]

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

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.

The result would be the conversion of (late type) low-luminosity spiral galaxies into dwarf spheroidals and dwarf ellipticals.[13]

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.[14] However, the existence of similar structures in isolated early-type dwarf galaxies, such as LEDA 2108986, has undermined this hypothesis[15][16]

Notable interacting galaxies
Montage of some well known interacting galaxies
Name Type Distance
(million ly)		 Magnitude Notes
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
NGC 2936 Irr +12.9 ?

Future collision of the Milky Way with Andromeda
Main article: 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 galaxies will eventually merge to become an elliptical galaxy[17][18] or perhaps a large disk galaxy.[19]

See also

References
  1. "HubbleSite: News - Hubble Finds that "Blue Blobs" in Space Are Orphaned Clusters of Stars". hubblesite.org. Retrieved 2017-05-24.
  2. "Best View Yet of Merging Galaxies in Distant Universe". ESO Press Release. Retrieved 26 August 2014.
  3. space.com 2015-04-21 How the Hubble Space Telescope Changed Our View of the Cosmos-Galactic Collisions Photographs
  4. GALMER 27 March 2010
  5. "Galactic Creatures at Play". www.spacetelescope.org. Retrieved 10 August 2019.
  6. "Close encounter". www.spacetelescope.org. Retrieved 8 May 2017.
  7. "A close galactic pair". www.spacetelescope.org. Retrieved 21 April 2017.
  8. "Two become one". Retrieved 28 December 2015.
  9. "Galactic soup". ESA/Hubble Picture of the Week. Retrieved 18 August 2014.
  10. "The messy result of a galactic collision". ESA/Hubble Picture of the Week. Retrieved 29 May 2013.
  11. "Defying cosmic convention". www.spacetelescope.org. Retrieved 20 March 2017.
  12. "The last waltz". Retrieved 14 December 2015.
  13. Galaxy Harassment
  14. More evidence for hidden spiral and bar features in bright early-type dwarf galaxies
  15. Graham, A.W. et al. (2017), 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 SK2 Kinematic Scaling, and the Spin-Ellipticity Diagram
  16. Janz, J. et al. (2017), Implications for the origin of early-type dwarf galaxies - the discovery of rotation in isolated, low-mass early-type galaxies
  17. whose gravitational interactions will fling various celestial bodies outward, evicting them from the resulting elliptical galaxy.Hazel Muir (2007-05-14). "Galactic merger to 'evict' Sun and Earth". New Scientist. Archived from the original on 20 April 2014. Retrieved 2014-10-07.
  18. Astronomy, June 2008, page 28, by Abraham Loeb and T.J.Cox
  19. Junko Ueda; et al. (2014). "Cold molecular gas in merger remnants. I. Formation of molecular gas disks". The Astrophysical Journal Supplement Series. 214 (1): 1. arXiv:1407.6873. Bibcode:2014ApJS..214....1U. doi:10.1088/0067-0049/214/1/1.
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