Galaxy merger
Encyclopedia
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
collide. They are the most violent type of galaxy interaction. Although galaxy mergers do not involve stars or star systems actually colliding, due to the vast distances between stars in most circumstances, the gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. There are some generally accepted results, however:
- When one of the galaxies is significantly larger than the other, the larger will often "eat" the smaller, absorbing most of its gas and stars with little other major effect on the larger galaxy. Our home galaxy, the Milky WayMilky WayThe Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
, is thought to be currently absorbing smaller galaxies in this fashion, such as the Canis Major Dwarf GalaxyCanis Major Dwarf GalaxyThe Canis Major Dwarf Galaxy is located in the same part of the sky as the constellation Canis Major. The galaxy contains a relatively high percentage of red giant stars, and is thought to contain an estimated one billion stars in all....
, and possibly the Magellanic CloudsMagellanic CloudsThe two Magellanic Clouds are irregular dwarf galaxies visible in the southern hemisphere, which are members of our Local Group and are orbiting our Milky Way galaxy...
. The Virgo Stellar StreamVirgo Stellar StreamThe 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 process of merging with the Milky Way...
is thought to be the remains of a dwarf galaxyDwarf galaxyA dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars...
that has been mostly merged with the Milky Way.
- If two spiral galaxies that are approximately the same size 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 are active galactic nuclei. This is thought to be the driving force behind many quasars. The end result is an elliptical galaxyElliptical galaxyAn elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
, and many astronomers hypothesize that this is the primary mechanism that creates ellipticals.
Note that the Milky Way and the Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
will probably collide in about 4.5 billion years. If these galaxies merged, the result would quite possibly be an elliptical galaxy as described above.
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.
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 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. Such a library of galaxy merger simulations can be found on the GALMER website
Examples
Some galaxies that are suspected to be in the process of merging:- Antennae GalaxiesAntennae GalaxiesThe Antennae Galaxies are a pair of interacting galaxies in the constellation Corvus. They are currently going through a phase of starburst. They were discovered by William Herschel in 1785...
- Mice GalaxiesMice GalaxiesNGC 4676, or the Mice Galaxies, are two spiral galaxies in the constellation Coma Berenices. About 290 million light-years away, they are presently in the process of colliding and merging...
- Centaurus ACentaurus ACentaurus A is a prominent galaxy in the constellation of Centaurus. There is considerable debate in the literature regarding the galaxy's fundamental properties such as its Hubble type and distance...
- NGC 7318NGC 7318NGC 7318 are a set of galaxies about 300 million light-years away in the Constellation Pegasus...
See also
- Andromeda-Milky Way collisionAndromeda-Milky Way collisionThe collision seen from a hypothetical extragalactic planet. The galaxies will be at this relative angleThe Andromeda–Milky Way collision is a predicted galaxy collision that could possibly take place in approximately 3 to 5 billion years' time between the two largest galaxies in the Local...
- Bulge (astronomy)Bulge (astronomy)In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies...
- Galaxy formation and evolutionGalaxy formation and evolutionThe 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 generated the variety of structures observed in nearby...
- Interacting galaxies
- Pea galaxyPea galaxyA Pea galaxy, also referred to as a Pea or Green Pea, is a type of Luminous Blue Compact Galaxy which is undergoing very high rates of star formation...