Interacting galaxies
Interacting galaxies are galaxies that influence each other and thus increase their internal activities.
Interactions occur when two or more galaxies meet. This results in either galaxy mergers or special formations and new constellations of the star islands involved.
Merging of galaxies, especially of protogalaxies, takes place preferentially in dense regions of the universe at slow relative speeds to each other. At faster collision speeds, the units penetrate and cross each other. In some cases, galaxies drift past each other at a short distance. Elliptical galaxies are mostly the merging products of disk galaxies, especially spiral galaxies .
Today there are only about one to two out of a hundred galaxies in the direct process of merging. There are many indications that in the period around a billion years after the Big Bang the dwarf galaxies, which were common at the time, merged with one another many times.
The cosmological hierarchy
According to current knowledge, the content structure of the cosmos has developed hierarchically; from small to large. Accordingly, galaxies first formed, then galaxy groups , galaxy clusters and galaxy superclusters. Today, galaxy clusters and superclusters are developing even more clearly.
The starting material of all stars and galaxies is interstellar gas . This gas initially forms protogalaxies. Protogalaxies are pre-forms of galaxies, and they have hardly formed any stars. In young galaxy clusters, most of the gas is found in irregular accumulations between the galaxies in the cluster. Well-developed galaxy clusters, on the other hand, show a uniform distribution of the gas; uniform, symmetrical and concentrated towards the center of the cluster. A glance into distant galaxies, i.e. into the past, is not enough to show the situation a billion years after the Big Bang .
Nevertheless, larger galaxies must have gradually formed from a great many dwarf galaxies in the small hierarchical level . These larger galaxies were or are then, like our Milky Way system today , surrounded by several dwarf galaxies. During these galaxy formations, large parts of the original gas remained as remnants, as did numerous weak galaxies. In the meantime, many such extremely faint dwarf galaxies have been found, which are very close but can hardly be seen in the night sky.
Dwarf galaxies usually have irregular shapes. The first tier of larger galaxies are pure bar galaxies . The bars are created by tidal forces, more precisely by differences in gravitational impulses. These formations then develop into increasingly pronounced spirals . The bar eventually becomes a disc-shaped, elliptical or spherical center. Bar and spiral galaxies rotate. Their shape results from the rotation of the stars. The rotational movement exists because the galaxies were formed from turbulent gas clouds and / or dwarf galaxies. The speed energies of these building blocks have then standardized to form a uniform rotation.
With the help of computer simulations it was possible to show that the union of two gas-rich spiral galaxies, the most common type of galaxy, typically produces elliptical galaxies . The stars of an elliptical galaxy do not only move in one plane, as in bar and spiral galaxies. They have elliptical and randomly oriented orbits. The shape of elliptical galaxies can roughly be explained by the fact that two disk-shaped objects almost always meet with an offset angle of their planes. This results in a galaxy shape that is no longer concentrated on just one plane.
Tidal forces
Tidal forces accelerate and unify the processes of star and galaxy formation. These forces can exert very different influences in the various constellations of interacting galaxies.
Outside the galaxy clusters, where tidal forces have less influence, the gas has collapsed comparatively calmly over the course of more than ten billion years. There the star formation and the sequence of star populations took place step by step. The collisions and mergers of galaxies there also took place more slowly and gradually. Elliptical galaxies are mostly found in dense galaxy clusters. According to this, elliptical galaxies formed under the strong influence of tidal forces, presumably from existing disk galaxies or from gas-rich protogalaxies.
Collision phases and galaxy mergers
Merging galaxies take several hundred million years to 1.5 billion years, the calming processes take much longer. The objects first approach each other and circle around each other. Here it is important how big or how heavy the galaxies are compared to each other. The reference points for the circular paths are the centers (cores) of the star islands. The orbits then become increasingly narrow. In most cases, the galaxies penetrate each other several times before they finally merge. Through mutual penetration, the original forms are broken and matter such as gas and stars are exchanged. Gravity must be enough to hold the torn galaxies together. In the other case, after the first penetration, they drift apart.
- Simulation of an encounter between two galaxies
Approach phase
A hit
Self-gravity and pause
- Approach phase (pre-collision): In the left image of the simulation, the approach phase of two disk-shaped galaxies can be seen. Approach speeds are in the dimensions of below 100 km / s up to over 1,000 km / s in extreme cases. When core regions of two galaxies collide, however, speeds of 3,000 km / s and more can exist. The inner structures of the two galaxies change as they approach, and gas and star bridges can already exist between them when they approach. The galaxy cores describe a parabolic course as they approach.
- Impact: The picture in the middle shows how the two objects roam at their edges. The galaxy cores do not meet here. The galaxies deform and exchange material. At their closest point to each other, the tidal force of the other collision partner acts on each galaxy. As a result of this impulse, tidal arms form on their sides facing away from each other (right picture). The interstellar matter is condensed there by shocks and currents in the galaxy disks.
- Self-gravity (Gravitational Response) and pause: In the picture on the right, the original galaxies have changed both in their external forms and in their internal processes, and the mass ratios of the collision partners involved may have shifted significantly. The galaxies are initially moving away from each other. The so-called self - gravity of the galaxy disks can then generate or reinforce spiral arms and / or bars, which can be a possible response to the previous compression processes. However, how the objects deform depends heavily on the internal structures of the galaxies and their trajectories to one another. When drifting apart, the collision partners then first enter the pause phase . It now depends on the mass and speed ratios whether the galaxies will move towards each other again later, which would ultimately lead to a merger, or whether they will finally drift apart after this first meeting.
- Merging phase (English. Merging): The movements of the galaxies to each other get in ever more circular and narrow orbits, which intensifies the collisions of the gas clouds. From this, a dense gas cloud forms in the center under high pressure. This cloud becomes unstable due to the high pressure and collapses. The gas forms new stars. In this way, an extremely large number of stars are formed, which in this form is called a starburst . Much of the gas is blown out of the star system by the tremendous energies of the new stars. There are many stars and little gas left. As a result, no or almost no new stars can emerge later in these galaxies. A galaxy newly formed in this way consists of stars that age together, with hardly any younger stars following. When larger galaxies merge, mostly elliptical galaxies are created (see section on the cosmological hierarchy ). In today's elliptical galaxies there are almost exclusively very old stars of the same generation ( star population ) with a mass similar to that of the sun. In other types of galaxies there are both different star populations and higher amounts of gas.
- Relaxation phase : After the union, the newly formed galaxy has to go through several periods of rotation until an equilibrium is established. This period is relatively long compared to the previous processes. In the galaxy centers, the time spans are only around a hundred million years, but in the outer regions they are several billion years.
When galaxies collide or merge, the stars of the galaxies drift past each other. Collisions between stars are very unlikely because they only take up an extremely small fraction of the space. The gas clouds collide, however, and lose orbital energy in the process. This creates many globular clusters that are evenly distributed as a sphere in most older galaxies, which have usually already gone through several collision processes. These spherical structures partly consist of older star clusters that existed before the last collision and newer star clusters that were formed during the most recent collision.
Polar ring galaxies
Polar ring galaxies are very rare objects. Only about 100 specimens have been discovered to date. This type of galaxy is believed to be the result of the merging of a larger galaxy with a smaller one. In the case of such an object, an at least temporarily stable star ring has formed as a result of the merging. The ring is perpendicular to the plane of the main galaxy and rotates in its plane.
If a galaxy were formed from gas in a single collapse process, a gas disk with a preferred direction of rotation would develop. Participating gas clouds with a different direction of rotation would align themselves with the dominant direction of rotation through collisions. Therefore it is very unlikely that polar ring galaxies with their mutually perpendicular rotations of the ring and central galaxy were formed in one go as individual galaxies.
One of the best studied polar ring galaxies is NGC 4650A. It is about 150 million light years from Earth and is often considered the prototype of the class. The central part contains older yellowish stars. A much larger ring (actually a wide disk rather than a thin ring) with younger blue stars rotates almost perpendicular to it.
Currently interacting galaxies
The light of very distant galaxies reaches us from the deepest past. The proportion of galaxies with strongly distorted shapes from this time is much higher than in our region of the cosmos. These more common irregular shapes are the result of interactions and fusions. As a result, such processes were correspondingly more frequent in the past. Near us, the proportion is only one to two percent.
The following constellations result in a sequence, arranged according to the stage of collision and union.
The Hickson Compact Group 87 (approximation phase)
The Hickson Compact Group 87 (HCG 87) is formed from the neighboring galaxies HCG 87a, 87b, and 87c. They are about 400 million light years away from us and are in the constellation Capricorn . The entire expanse of the group is 170,000 light years.
They are galaxies that are moving towards each other. They are so close to each other that the mutual gravitational interactions tear their inner structures apart. All three group members show high star formation rates.
The largest of the three is the spiral galaxy 87a (left in the picture). It is closest to the elliptical galaxy 87b (right). 87a and 87b both have active nuclei in which a black hole is suspected. A lively gas exchange takes place between these two objects and there is a star bridge. The gas exchange between the participating galaxies is increasing. It intensifies the activity of their core regions, which results in the formation of new stars. 87c (pictured above) is a smaller spiral galaxy.
The small dark object in the center of the image is also a spiral galaxy. It has not yet been possible to find out whether she is a fourth member of the group or an independent background galaxy. The two bright points in the middle of the picture do not belong to the group. They are objects that happened to be in the field of view of the Hubble Space Telescope .
Presumably, these galaxies will circle around each other at ever closer distances and penetrate each other several times. The result will be the merging of all three galaxies into a single elliptical galaxy. The process will take at least several hundred million years.
NGC 2207 and IC 2163 (impact phase)
NGC 2207 (left) and IC 2163 (right) are both pure spiral galaxies without bars. You are 144 million light years away in the constellation of the Big Dog . These two star formations are already in the process of colliding, but in contrast to NGC 4676 and the Antennae Galaxy, they are still two completely separate galaxies. The first meeting is just beginning. In the further course they will initially assume a shape similar to that of NGC 4676 and finally approximate the appearance of the antenna galaxies. A large elliptical galaxy will have emerged from them over a period of about a billion years.
NGC 2207 and IC 2163 were both discovered by John Herschel in 1835 . Two supernovae could already be observed in NGC 2207 . The Hubble recording was made in 1999.
NGC 4676, "the mice" (self-gravity phase)
NGC 4676 (“the mice”), or individually IC 820 (left) and IC 819 (right), is a pair of spiral galaxies. It is about 300 million light years away and is in the constellation Coma Berenices . Its nickname comes from the long tails, which practically represent the mouse tails. These tails are a result of the action of the tides, i.e. they result from the relative difference in attraction between the near and distant parts of the two galaxies.
NGC 4676 is in the self-gravity phase. The structure shows that the two galaxies have already passed each other. Their centers, however, have not yet touched or penetrated each other as in the case of the antenna galaxy, the next memorable step in the merging process.
The antenna galaxy (merging phase)
The antenna galaxy was discovered by William Herschel on February 7, 1785 . It is 68 million light years away and is in the constellation Raven . After their English name "Antennae Galaxies" there are still two galaxies. If you take its German name “Antennen-Galaxie” as a yardstick, it is no longer two galaxies, but only one. In its original English name, the word "Antennae" means "feeler". It got its name because of its thin upturned tails (see distance view in the picture above left), which are reminiscent of the antennae of insects.
The cores of the colliding galaxies have already penetrated each other here. As the two galaxies merge, the interstellar gas in particular is compressed and star formation is stimulated. The star formation areas and the associated emission nebulae are visible as bright nodes in the spiral arms.
The galaxy NGC 4038 (left) used to be a spiral galaxy and NGC 4039 (right) was a bar-shaped spiral. Before their first meeting, which took place about 900 million years ago, the barred spiral NGC 4039 (right) was the larger galaxy. It has now become a smaller part of the system. 600 million years ago the process was in the present stage of NGC 4676. The feelers were created about 300 million years ago. In about 400 million years a common stable core will have formed, as it exists today in the starfish galaxy.
The starfish galaxy (calming phase)
The starfish galaxy (NGC 6240, IC 4625, UGC 10592) is approximately 400 million light years away and is in the constellation of Serpent Bearer .
It is the result of the collision and merging of two galaxies. However, the merging process is not yet complete and no regular structure such as an ellipse or spiral has formed. In the center of the starfish galaxy, two active supermassive black holes orbit each other 3,000 light years apart. They were the nuclei of the two starting galaxies. The two black holes emit a large amount of X-rays . They will only merge in a few hundred million years.
The starfish galaxy was discovered on July 29, 2001 using the Chandra X-ray telescope. It was the first galaxy to have two supermassive black holes found in the core. This system is a prime example of a starburst galaxy with a high rate of star formation.
Ring galaxies
Occasionally, galaxies with ring structures are formed. In ring galaxies , one galaxy has been pierced by another galaxy. A compression wave creates an outwardly running blue ring in the plane of the galaxy. In contrast to polar ring galaxies, ring galaxies are unstable.
Probably the best known ring galaxy is the wagon wheel galaxy in the constellation Sculptor. At 500 million light years, it is relatively distant and no longer listed in the NGC catalog . The ring of the wagon wheel galaxy is 150,000 light years in diameter. It is unstable and moves away from the center at 340,000 km / h (94 km / s).
Interactive satellite galaxies
It is relatively common for a larger galaxy to incorporate a dwarf galaxy. The time span from the first touch of the spiral arms to the complete incorporation into the center is about 900 million years. Such a first touch of the spiral arms can currently be found in the well-known Whirlpool galaxy and its satellite galaxy.
The Whirlpool Galaxy (also Messier 51 or NGC 5194/5195) in the constellation of the Hounds (Canes Venatici) is about 31 million light years away. It is a well-known spiral galaxy of the Hubble type Sc , that is, with a clearly pronounced spiral structure. The interactive companion of the Whirlpool galaxy is of an irregular type. In the NGC catalog it bears the number NGC 5195 (M51 itself has the number NGC 5194).
Exceptionally active star formation is currently taking place in M51, presumably caused by the tidal interaction with NGC 5195. Therefore the entire galaxy has a high proportion of young and massive stars. Such massive, hot stars typically have a very short lifespan of only a few million to a few dozen million years.
The center of the Whirlpool galaxy, the active galactic core , is strikingly hot. There are stellar winds , expanding supernova remnants and the accretion of matter into the central black hole. There are also numerous collisions between gases.
Also in the spiral arms of M51 are active star formation areas and thus many young hot stars. The star formation area in the spiral arm between M51 and the companion is particularly pronounced in this regard.
Investigation methods for interacting galaxies
The study of interacting galaxies falls within the scope of extragalactic astronomy . A wide range of astronomical methods is used here.
The most important sub-area is called astro spectroscopy . Astrospectroscopy is the wavelength-dependent analysis of the radiation from astronomical objects , which also includes interacting galaxies and their components such as galaxy nuclei, gas nebulae, stars and supernovae . These analyzes are divided according to wavelengths into infrared astronomy , radio astronomy , ultraviolet astronomy , X-ray astronomy , gamma astronomy and the range of visible light. One also works with multispectral analyzes and overexposure. By redshift in radiation spectra differences in distance can be determined from galaxies. One orientates oneself on so-called astronomical standard candles .
In the spectrum of ultraviolet light, it is particularly easy to make out high star formation rates. Radio astronomy, on the other hand, is the best way to study active galactic nuclei , which are very pronounced in radio galaxies and Seyfert galaxies . Active galactic nuclei emit an unusually large amount of radio emissions. Large elliptical galaxies with active galactic nuclei (see Starfish Galaxie) often arise through galaxy mergers and the connection between other interactions and the activities of galaxy centers is an interesting subject of investigation (see Whirlpool Galaxy). On a radio map of an elliptical galaxy, for example, you can see radio bubbles that are many times larger than the optically visible part of the light.
However, if one compares a normally exposed image with a strongly overexposed image in the optical range of elliptical galaxies, one also recognizes a structure that is many times larger than the object with normal exposure: the gas that is released when the galaxies merge through the star formation (see section on the merging phase ) is driven out of the system, extends over a large area around the newly formed galaxy.
The two images on the right are images from the Spitzer telescope . Both images show the same view of the galaxy group Stephen's Quintet in the constellation Pegasus, about 300 million light-years away . Stephen's quintet is the most studied of all compact galaxy groups. So far one has been able to discover an exceptionally large number of special phenomena in this group. This extraordinary constellation is very turbulent and allows conclusions to be drawn about events that took place in the relatively young universe about 10 billion years ago.
For example, in the image above, you can see the largest shock wave ever observed. Four of the five galaxies are on a collision course. The rapidly falling galaxy NGC 7318B (middle right in the picture, left double eye) causes a shock wave of 870 km / s, which can be seen as a green train in the middle of the picture above. The wavefront is larger than our home galaxy.
In the upper image, several spectra are superimposed: X-ray, infrared and radio radiation, with visible light added. The components each serve to make certain things visible. The shock wave is actually invisible to our eyes (see picture below). The Spitzer telescope can detect infrared radiation from normally invisible objects such as grains of dust or hydrogen molecules. Along its movement, the wave excites hydrogen molecules to emit infrared radiation, which outlines the shape of the wave.
The infrared emissions (area shown in green) can be used to calculate how fast the wave is moving: In spectroscopy, the emissions of a gas, in this case hydrogen, can be broken down into its spectral components. By moving forwards or forwards, as is the case with the shock wave, these components (spectral lines) shift depending on the relative speed. These shifts in the spectral lines are called Doppler broadening . Since they are speed-dependent, they show the relative speed of the wave.
See also
literature
- Joseph Silk : The History of the Cosmos . Spectrum Academic Publishing House, Heidelberg 1999. ISBN 3-8274-0482-7 .
- Roy A. Gallant: Our Universe . Weltbildverlag, Augsburg 1998. ISBN 3-8289-3391-2 .
- Simon Goodwin: Mission Hubble . Bechtermünz Verlag / Weltbildverlag, Augsburg 1996. ISBN 3-86047-146-5 .
Web links
- Astronews - collision of two galaxies
- Wilhelm Foerster Observatory - Interactive Galaxies
- NASA article on Stephen's Quintet ( Memento from January 5, 2007 in the Internet Archive )
- 1 2 - Hubsite archive of Stephan's quintet
- Inastars - Interacting Galaxies NGC 3190 and NGC 3187
- Chandra Harvard - NGC 6240
- LISA - galaxy dynamics
- Galasimu - simulation of interacting galaxies
- Simone Bahia - Galaxy types and interacting galaxies
- Max Planck Society - Cosmic Evolution (PDF; 117 kB)
- NASA: Interacting Galaxies by Hubble - A collage of 59 images of merging galaxies - released on April 24, 2008 on the occasion of the 18th birthday of the Hubble Space Telescope
