TheMice Galaxies (NGC 4676 A&B) are in the process of merging.
Galaxy mergers can occur when two (or more)galaxies collide. They are the most violent type ofgalaxy interaction. Thegravitational interactions between galaxies and the friction between thegas anddust have major effects on the galaxies involved, but the exact effects of such mergers depend on a wide variety of parameters such ascollision angles,speeds, and relative size/composition, and are currently an extremely active area of research. Galaxy mergers are important because the merger rate is a fundamental measurement ofgalaxy evolution and also provides astronomers with clues about how galaxies grew into their current forms over long stretches of time.[1]
During the merger, stars anddark matter in each galaxy become affected by the approaching galaxy. Toward the late stages of the merger, thegravitational potential begins changing so quickly that star orbits are greatly altered, and lose any trace of their prior orbit. This process is called “violent relaxation”.[2] For example, when two disk galaxies collide they begin with their stars in an orderly rotation in the planes of the two separate disks. During the merger, that ordered motion is transformed into random energy (“thermalized”). The resultant galaxy is dominated by stars that orbit the galaxy in a complicated and random interacting network of orbits, which is what is observed inelliptical galaxies.
NGC 3921 is an interacting pair of disc galaxies in the late stages of its merger.[3]ESO 239-2, an interacting pair of galaxies located 550 million light-years in the constellation of Grus. These galaxies are currently in the last stages of merging which the end result would be anelliptical galaxy.[4]
Mergers are also locations of extreme amounts ofstar formation.[5][6] 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 itsredshift.[7][8] Typical merger SFRs are less than 100 new solar masses per year.[9][10] This is large compared to our Galaxy, which makes only a few new stars each year (~2 new stars).[11] Though stars almost never get close enough to actually collide in galaxy mergers, giantmolecular clouds rapidly fall to the center of the galaxy where they collide with other molecular clouds.[citation needed][12] 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 moremolecular 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 (seeStellar 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 isquenched.[13][citation needed]
SDSSCGB 10389, a trio of galaxies merging into a single object[14]Example of computer simulation of Milky Way and Andromeda collision[15]
Galaxy mergers can be simulated in computers, to learn more about galaxy formation. One famous example of this simulation is probably from the merging of ourMilky Way andAndromeda. The computer simulations using the information given by Hubble indicate that two billion years following the collision, the interacting galaxies will finish entirely fusing under the influence of gravity and transform into one elliptical galaxy resembling the ones frequently found in the local universe.[16]Galaxy pairs initially of any morphological type can be followed, taking into account allgravitational forces, and also thehydrodynamics anddissipation of the interstellar gas, the star formation out of the gas, and the energy and mass released back in the interstellar medium bysupernovae. Such a library of galaxy merger simulations can be found on the GALMER website.[17] A study led byJennifer Lotz of theSpace Telescope Science Institute inBaltimore, Maryland created computer simulations in order to better understand images taken by theHubble Space Telescope.[1] Lotz's team tried to account for a broad range of merger possibilities, from a pair of galaxies with equal masses joining to an interaction between a giant galaxy and a tiny 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.[1]
One of the largest galaxy mergers ever observed consisted of fourelliptical galaxies in the cluster CL0958+4702. It may form one of the largest galaxies in the Universe.[18]
Galaxy mergers can be classified into distinct groups due to the properties of the merginggalaxies, such as their number, their comparative size and theirgas richness.
Mergers can be categorized by the extent to which the largest involved galaxy is changed in size or form by the merger:
Minor merger
A merger isminor if one of thegalaxies is significantly larger than the other(s). The larger galaxy will often "eat" the smaller - a phenomenon aptly named “galactic cannibalism” - absorbing most of its gas and stars with little other significant effect on the larger galaxy. Our home galaxy, theMilky Way, is thought to be currently absorbing several smaller galaxies in this fashion, such as theCanis Major Dwarf Galaxy, and possibly theMagellanic Clouds. TheVirgo Stellar Stream is thought to be the remains of adwarf galaxy that has been mostly merged with the Milky Way.
Major merger
A merger of twospiral galaxies that are approximately the same size ismajor; if they collide at appropriate angles and speeds, they will likely merge in a fashion that drives away much of the dust and gas through a variety of feedback mechanisms that often include a stage in which there areactive galactic nuclei. This is thought to be the driving force behind manyquasars. The result is anelliptical galaxy, and many astronomers hypothesize that this is the primary mechanism that creates ellipticals.
Mergers can be categorized by the degree to which the gas (if any) carried within and around the merging galaxies interacts:
Wet merger
Awet merger is between gas-rich galaxies ("blue" galaxies). Wet mergers typically produce a large amount of star formation, transform discgalaxies intoelliptical galaxies and triggerquasar activity.[19]
Dry merger
A merger between gas-poor galaxies ("red" galaxies) is calleddry. Dry mergers typically do not greatly change the galaxies'star formation rates, but can play an important role in increasingstellar mass.[19]
Damp merger
Adamp merger occurs between the same two galaxy-types mentioned above ("blue" and "red" galaxies), if there is enough gas to fuelsignificant star formation but not enough to formglobular clusters.[20]
Mixed merger
Amixed merger occurs when gas-rich and gas-poor galaxies ("blue" and "red" galaxies) merge.
In the standard cosmological model, any single galaxy is expected to haveformed from a few or many successive mergers ofdark matter haloes, in which gas cools and forms stars at the centres of the haloes, becoming the optically visible objects historically identified as galaxies during the twentieth century. Modelling themathematical graph of the mergers of these dark matter haloes, and in turn, the corresponding star formation, was initially treated either by analysing purely gravitationalN-body simulations[21][22] or by using numerical realisations of statistical ("semi-analytical") formulae.[23]
In a 1992 observational cosmology conference inMilan,[21] Roukema,Quinn andPeterson showed the first merger history trees of dark matter haloes extracted from cosmologicalN-body simulations. These merger history trees were combined with formulae for star formation rates and evolutionary population synthesis, yielding synthetic luminosity functions of galaxies (statistics of how many galaxies are intrinsically bright or faint) at different cosmological epochs.[21][22] Given the complex dynamics of dark matter halo mergers, a fundamental problem in modelling merger history tree is to define when a halo at one time step is a descendant of a halo at the previous time step. Roukema's group chose to define this relation by requiring the halo at the later time step to contain strictly more than 50 percent of the particles in the halo at the earlier time step; this guaranteed that between two time steps, any halo could have at most a single descendant.[24] This galaxy formation modelling method yields rapidly calculated models of galaxy populations with synthetic spectra and corresponding statistical properties comparable with observations.[24]
Independently, Lacey andCole showed at the same 1992 conference[25] how they used thePress–Schechter formalism combined withdynamical friction to statistically generate Monte Carlo realisations of dark matter halo merger history trees and the corresponding formation of the stellar cores (galaxies) of the haloes.[23]Kauffmann,White and Guiderdoni extended this approach in 1993 to include semi-analytical formulae for gas cooling, star formation, gas reheating from supernovae, and for the hypothesised conversion of disc galaxies into elliptical galaxies.[26] Both the Kauffmann group and Okamoto and Nagashima later took up theN-body simulation derived merger history tree approach.[27][28]
^Schweizer, F. (2005). de Grijs, R.; González-Delgado, R.M. (eds.).[no presentation title cited]. Starbursts: From 30 Doradus to Lyman Break Galaxies; Cambridge, UK; 6–10 September 2004. Astrophysics & Space Science Library. Vol. 329. Dordrecht, DE: Springer. p. 143.[full citation needed]
^Starbursts : from 30 Doradus to Lyman break galaxies. Richard De Grijs, Rosa M. González Delgado. Dordrecht: Springer. 2005. p. 143.ISBN978-1-4020-3539-5.OCLC262677690.{{cite book}}: CS1 maint: others (link)
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