Galaxy mergers can occur when two (or more)

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

and

dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in homes ...

have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety of parameters such as collision angles,

speed In everyday use and in kinematics, the speed (commonly referred to as ''v'') of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a scalar quanti ...

s, 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 of

galaxy evolution The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have gen ...

. The merger rate also provides astronomers with clues about how galaxies bulked up over time.

Description

During the merger, stars and

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...

in each galaxy become affected by the approaching galaxy. Toward the late stages of the merger, the gravitational potential (i.e. the shape of the galaxy) begins changing so quickly that star orbits are greatly altered, and lose any trace of their prior orbit. This process is called “violent relaxation”. For example, when two disk galaxies collide they begin with their stars in a 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 in elliptical galaxies. Evolution in slow motion

Mergers are also locations of extreme amounts of

star formation Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

. 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 its redshift. Typical merger SFRs are less than 100 new solar masses per year. This is large compared to our Galaxy, which makes only a few new stars each year (~2 new stars). Though stars almost never get close enough to actually collide in galaxy mergers, giant molecular clouds rapidly fall to the center of the galaxy where they collide with other molecular clouds. 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 more molecular 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 (see

Stellar evolution Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...

) 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 is quenched. Galaxy mergers can be simulated in computers, to learn more about galaxy formation. Galaxy pairs initially of any morphological type can be followed, taking into account all gravitational forces, and also the

hydrodynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) and ...

and dissipation of the interstellar gas, the star formation out of the gas, and the energy and mass released back in the interstellar medium by

supernovae A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when a ...

. Such a library of galaxy merger simulations can be found on the GALMER website. A study led by

Jennifer Lotz Jennifer Mae Lotz is an American astronomer who studies the shape and evolution of galaxies, including galaxy mergers. She works at the NOIRLab, a project of the National Science Foundation, as director of the Gemini Observatory. Early life and ...

of the Space Telescope Science Institute in

Baltimore, Maryland Baltimore ( , locally: or ) is the List of municipalities in Maryland, most populous city in the U.S. state of Maryland, fourth most populous city in the Mid-Atlantic (United States), Mid-Atlantic, and List of United States cities by popula ...

created computer simulations in order to better understand images taken by the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

. Lotz's team tried to account for a broad range of merger possibilities, from a pair of galaxies with equal masses joining together 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. One of the largest galaxy mergers ever observed consisted of four elliptical galaxies in the cluster CL0958+4702. It may form one of the largest galaxies in the Universe.

Categories

Galaxy mergers can be classified into distinct groups due to the properties of the merging

, such as their number, their comparative size and their

richness.

By number

Mergers can be categorized by the number of galaxies engaged in the process: ;Binary merger: Two interacting galaxies merge. ;Multiple merger: Three or more galaxies merge.

By size

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 is ''minor'' if one of the

is significantly larger than the other(s). The larger galaxy will often "eat" the smaller, absorbing most of its gas and stars with little other significant effect on the larger galaxy. Our home galaxy, the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

, is thought to be currently absorbing several smaller galaxies in this fashion, such as the

Canis Major Dwarf Galaxy The Canis Major Overdensity (''CMa Overdensity'') or Canis Major Dwarf Galaxy (''CMa Dwarf'') is a disputed dwarf irregular galaxy in the Local Group, located in the same part of the sky as the constellation Canis Major. The supposed small gal ...

, and possibly the Magellanic Clouds. The

Virgo Stellar Stream The Virgo Stellar Stream, also known as Virgo Overdensity, is the proposed name for a stellar stream in the constellation of Virgo which was discovered in 2005. The stream is thought to be the remains of a dwarf spheroidal galaxy that is in the pr ...

is thought to be the remains of a dwarf galaxy that has been mostly merged with the Milky Way. ;Major merger: A merger of two

spiral galaxies Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''active galactic nuclei. This is thought to be the driving force behind many quasars. The end result is an elliptical galaxy, and many astronomers hypothesize that this is the primary mechanism that creates ellipticals. One study found that large galaxies merged with each other on average once over the past 9 billion years. Small galaxies coalesced with large galaxies more frequently. Note that the

and the Andromeda Galaxy are predicted to collide in about 4.5 billion years. The expected result of these galaxies merging would be ''major'' as they have similar sizes, and will change from two "grand design" spiral galaxies to (probably) a giant elliptical galaxy.

By gas richness

Mergers can be categorized by the degree to which the gas (if any) carried within and around the merging galaxies interacts: ;Wet merger: A ''wet merger'' is between gas-rich galaxies ("blue" galaxies). Wet mergers typically produce a large amount of star formation, transform disc

into elliptical galaxies and trigger

quasar A quasar is an extremely Luminosity, luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a m ...

activity. ;Dry merger: A merger between gas-poor galaxies ("red" galaxies) is called ''dry''. Dry mergers typically do not greatly change the galaxies'

star formation Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

rates, but can play an important role in increasing

stellar mass Stellar mass is a phrase that is used by astronomers to describe the mass of a star. It is usually enumerated in terms of the Sun's mass as a proportion of a solar mass (). Hence, the bright star Sirius has around . A star's mass will vary over its ...

. ;Damp merger: A ''damp merger'' occurs between the same two galaxy-types mentioned above ("blue" and "red" galaxies), if there is enough gas to fuel significant star formation but not enough to form globular clusters. ;Mixed merger: A ''mixed merger'' occurs when gas-rich and gas-poor galaxies ("blue" and "red" galaxies) merge.

Merger history trees

In the standard cosmological model, any single galaxy is expected to have formed from a few or many successive mergers of

dark matter halo According to modern models of physical cosmology, a dark matter halo is a basic unit of cosmological structure. It is a hypothetical region that has decoupled from cosmic expansion and contains gravitationally bound matter. A single dark matte ...

es, 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 the mathematical graph of the mergers of these dark matter haloes, and in turn, the corresponding star formation, was initially treated either by analysing purely gravitational ''N''-body simulations or by using numerical realisations of statistical ("semi-analytical") formulae. In a 1992 observational cosmology conference in

Milan Milan ( , , Lombard: ; it, Milano ) is a city in northern Italy, capital of Lombardy, and the second-most populous city proper in Italy after Rome. The city proper has a population of about 1.4 million, while its metropolitan city h ...

, Roukema, Quinn and Peterson showed the first merger history trees of dark matter haloes extracted from cosmological ''N''-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. 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. This galaxy formation modelling method yields rapidly calculated models of galaxy populations with synthetic spectra and corresponding statistical properties comparable with observations. Independently, Lacey and Cole showed at the same 1992 conference how they used the

Press–Schechter formalism The Press–Schechter formalism is a mathematical model for predicting the number of objects (such as galaxies, galaxy clusters or dark matter halos) of a certain mass within a given volume of the Universe. It was described in an academic paper by ...

combined with

dynamical friction In astrophysics, dynamical friction or Chandrasekhar friction, sometimes called ''gravitational drag'', is loss of momentum and kinetic energy of moving bodies through gravitational interactions with surrounding matter in space. It was first disc ...

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. Kauffmann,

White White is the lightest color and is achromatic (having no hue). It is the color of objects such as snow, chalk, and milk, and is the opposite of black. White objects fully reflect and scatter all the visible wavelengths of light. White on ...

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. Both the Kauffmann group and Okamoto and Nagashima later took up the ''N''-body simulation derived merger history tree approach.

Examples

Some of the

that are in the process of merging or are believed to have formed by merging are: * Antennae Galaxies *

Mice Galaxies NGC 4676, or the Mice Galaxies, are two spiral galaxies in the constellation Coma Berenices. About 290 million light-years distant, they have begun the process of colliding and merging. Their name refers to the long tails produced by tidal acti ...

* Centaurus A *

NGC 7318 NGC 7318 (also known as UGC 12099/UGC 12100 or HCG 92d/b) is a pair of colliding galaxies about 280 million light-years from Earth. They appear in the Constellation Pegasus and are members of Stephan's Quintet. The Spitzer Space Telescope rev ...

Gallery

References

External links

Andromeda involved in galactic collision
– NBC News
"GALMER: Galaxy Merger Simulations"
{{authority control Galaxies *Galaxy merger Impact events Articles containing video clips