Interacting galaxy

Interacting galaxies are galaxies that influence each other and thereby increase their internal activities.

On interactions occur in encounters two or more galaxies. This results in either galaxy mergers or special formations Neukonstellationen and the islands of stars involved.

Mergers of galaxies, especially of proto-galaxies, are preferably used in dense regions of the Universe at slow speeds relative to each other instead. At faster speeds collision penetrate and pass through the units. In some cases galaxies drive a short distance past each other. Elliptical galaxies are mostly the products of merging disk galaxies, especially spiral galaxies.

Today found only about one to two hundred galaxies in the direct process of a merger. It implies much to the fact that in time the then frequent dwarf galaxies often fused together about a billion years after the Big Bang.

The cosmological hierarchy

The content structure of the cosmos has evolved hierarchically according to present knowledge; from small to large. Accordingly, first galaxies, then clusters of galaxies, galaxy clusters and superclusters of galaxies have formed. Galaxy clusters and super - heap are now going to train even more apparent.

The starting material of all the stars and galaxies is interstellar gas. This gas is formed first proto-galaxies. Proto-galaxies are pre-forming galaxies, and they still have hardly formed stars. In young galaxy cluster is the largest part of the gas in irregular accumulations between the cluster galaxies. Contrast, well-developed clusters of galaxies exhibit a uniform distribution of the gas; uniform, symmetrical and concentrated to the center of the cluster out. A look at distant galaxies, so in the past, is not enough to show the situation a billion years after the Big Bang.

Nevertheless, at that time must have formed in the small hierarchy of many dwarf galaxies gradually larger galaxies. These larger galaxies are or were then, as now, our Milky Way galaxy, surrounded by several dwarf galaxies. These galaxy formation, large parts of the original gas are left as remnants, as well as numerous faint galaxies. It has since found many such extremely weak dwarf galaxies that are very close, but in the night sky barely visible.

Dwarf galaxies usually have more irregular shapes. The first stage of larger galaxies are pure barred galaxies. The bars are caused by tidal forces, more precisely by differences of gravitational pulses. These formations develop into ever more distinctive spirals. The beam is finally to the disc-shaped, elliptical or spherical center. Bar and spiral galaxies rotate. Their shape is the result of the rotational movement of the star. The rotational motion is because the galaxies from turbulent gas clouds and / or dwarf galaxies have formed. The speed energies of these blocks have then standardized to uniform rotation.

Computer simulations could show you that emerge from groups of two gas-rich spiral galaxies, the most common existing types of galaxies, typically ellipticals. The stars of an elliptical galaxy not only move in a plane, such as for bar and spiral galaxies. You have elliptical and randomly oriented orbits. The shape of elliptical galaxies can be roughly explained by the fact that two disc-shaped objects almost always meet with an offset angle of their planes. This results in a galaxy form that is no longer focused only on one level.

Tidal forces

Accelerate tidal forces and standardized the processes of stars and galaxies formations. These forces can exercise in the various constellations of interacting galaxies very different influences.

Outside the galaxy clusters, where tidal forces have less influence, the gas is comparatively quiet collapsed in the course of over ten billion years ago. There the star formation and the sequence of stellar populations are gradually gone before him. The collisions and mergers of galaxies are run there slowly and gradually. Elliptical galaxies are found mostly in dense clusters of galaxies. Accordingly, 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

Mergers of galaxies take several hundred million years to 1.5 billion years, the settling processes take much longer. Here, the objects are approaching each other at first and circle each other. Here it plays a role in how large or how heavy the galaxies are compared to each other. The reference points for the orbits here are the centers ( nuclei ) of the islands of stars. The orbits are then closer and closer. In most cases the galaxies pass through it several times against each other before they finally merge. Through the mutual penetration of the original forms are broken and it will matter as gas and stars replaced. The force of gravity must be sufficient to hold the torn galaxy. In the other case they drift apart after the first penetration.

• Simulation of an encounter between two galaxies

• Approach phase (English Pre-Collision ): In the left image the simulation, the approach phase of two disk-shaped galaxies can be seen. Approach speeds are in the dimensions of less than 100 km / s to 1,000 km / s in extreme cases. In the collision of two galaxies core regions, however, speeds of 3,000 km / s or more may exist. When approaching the inner structures of the two galaxies change and there may already at the approach of gas and stars are bridges between them. The galaxy nuclei describe a parabolic trajectory when approaching.

• Impact ( engl. Impact): The picture in the middle shows how roam the two objects at their edges. The galaxy cores can not not here. The galaxies deform and exchange material. At its closest point to each other, the tidal force of the other collision partner acting on each galaxy. The momentum form on their sides facing away from each other tidal tails off ( right). The interstellar matter where it is compressed by shocks and flows in the disks of galaxies.

• Self- gravitation (English Gravitational Response) and Pause: In the right image, the original galaxies are changed both in their external forms as well as in the internal affairs and the mass ratios of the collision partners involved can have dramatically shifted. The galaxies are moving away from each other at first again. Through the so -called self- gravity of the disks of galaxies then spiral arms and / or bars can be created or strengthened, which may be a possible response to the preceding compaction processes. However strongly it depends on the internal structures of galaxies and their tracks extending from each other as to deform the objects. When drifting apart the collision partner takes you first to the pause phase (English break). It now depends on the mass and velocity ratios, whether the galaxies later move toward each other, which would eventually lead to a merger, or if they drift apart definitively after this first meeting.

• Merger phase (English Merging ): The movements of the galaxies fall to one another in ever more circular and closer orbits, which intensifies the collisions of gas clouds. It is formed under high pressure, a dense gas cloud in the center. This cloud becomes unstable by the high pressure and collapses. The gas forms new stars. In this way, extremely many stars, which in this form star-forming eruption (English starburst ) is called. A large proportion of the gas is bubbled through the enormous energies of the new star from the star system out. There remain many stars and little gas. This can later be long without or almost no new stars more in these galaxies. A galaxy in this way newly formed thus consists of common aging stars, with little more younger stars meet. With the merger of larger galaxies mostly elliptical galaxies form (see section on the cosmological hierarchy). In today's elliptical galaxies are therefore almost exclusively very old stars of the same generation ( stellar population ) with similar mass as the sun. In other types of galaxies both different stellar populations as well as higher amounts of gas are present.

• Settling phase (English relaxation): After the union of the newly formed galaxy has to go through several rotation periods, until an equilibrium is reached. This period is compared with the preceding process is relatively long. In the galaxy centers, the time periods be true only about a hundred million years in the outer regions but several billion years.

When galaxies collide or merge, the stars of the galaxies drive past each other. Collisions of stars are very unlikely because they occupy only a very small fraction of the space. However, the gas clouds collide and lose orbital energy. This results in many globular clusters, which are evenly distributed in most older galaxies that have undergone multiple collision processes usually considered sphere. These spherical structures are partly composed of older star clusters that existed before the last collision and newer star clusters that have emerged in the recent conflict.

Polar ring galaxies

Polar ring galaxies are very rare objects. It has been discovered to date only about 100 pieces. This type of galaxy is presumably formed by the fusion of a larger one with a smaller galaxy. In such an object has been through the fusion of an at least temporarily stable star ring formed. The ring is perpendicular to the main galaxy plane and rotates in its plane.

If a galaxy would arise from gas in a single collapse process, a gas disk would emerge with a preferred direction of rotation. Involved gas clouds with a different direction of rotation would align by collisions the dominant rotation. Therefore, it is very unlikely that polar ring galaxies have formed with their orthogonal rotations of the ring and central galaxy in a train than individual galaxies.

One of the best-studied polar ring galaxy NGC 4650A is. It is about 150 million light -years away from Earth and is often regarded as the prototype of the class. The central part contains older yellowish stars. Almost perpendicular to rotate a much larger ring (really a wide disc as a thin ring ) with younger blue stars.

Currently interacting galaxies

The light of very distant galaxies reaches us from the depths of the past. The fraction of galaxies with heavily distorted figures from this period is much higher than in our region of the universe. These frequent irregular shapes are the result of interactions and mergers. Consequently, such processes in the past were more frequently. In our neighborhood, the proportion is only one to two percent.

The following situations result in a sequence, ordered by the stage of collision and merging.

The Hickson Compact Group 87 ( approach phase )

The Hickson Compact Group 87 ( HGC 87) is formed by the neighboring galaxies HGC 87a, 87b, and 87c. They are about 400 million light years away from us and are located in the constellation Capricorn. The entire extent of the group spans 170,000 light years.

There are galaxies that move toward each other. You are so close to each other that the mutual gravitational interactions tear their internal structures apart. All three group members have high star formation rates.

The largest galaxy of the three is the spiral galaxy 87a ( left). You stand the elliptical galaxy 87b (right) the next. 87a and 87b both have active nuclei, in each of which a black hole is suspected. Between these two objects brisk gas exchange takes place and there is a star bridge. Gas exchange between the participating galaxies reinforced increasingly. He intensified the activity of their core regions, resulting in the formation of new stars for the episode. 87c ( top) is a smaller spiral galaxy.

The small dark object in the center of the picture is also a spiral galaxy. You could not figure out whether it is a fourth member of the group or an independent background galaxy so far. The two bright spots about the middle of the picture do not belong to the group. There are objects that happened in the field of vision of the Hubble Space Telescope.

Presumably, these galaxies are in orbit around each other ever closer intervals and thereby penetrate several times against each other. The result will be the fusion of all three galaxies into a single elliptical galaxy. The process will extend over 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 Canis Major. These two star formation are still two completely separate galaxies already in the process of collision, but in contrast to NGC 4676 and the Antennae Galaxies. It just starts the first meeting. They are in the further course first a shape similar to that of NGC 4676 and assume ultimately approach the appearance of the Antennae Galaxies. In the period of about a billion years will be developed from them a large elliptical galaxy.

NGC 2207 and IC 2163 have been discovered both in 1835 by John Herschel. In NGC 2207 we could already observe two supernovae. The Hubble image is from the year 1999.

NGC 4676, " the mice " ( self- gravitational phase)

NGC 4676 (the " mice " ), or individually IC 820 (left) and IC 819 ( right), a pair of spiral galaxies. It is about 300 million light years away, in the constellation Coma Berenices. His nickname comes from the long tails, which are virtually the mice tails. These tails are a result of the tidal effect, ie result from the relative difference of attraction to each other near and distant parts of the galaxy.

NGC 4676 is located in the self- gravity phase. The structure shows that the two galaxies are already over striped together. Their centers have not yet been touched or penetrated as the antenna galaxy, the next step in the merger process memorable.

The Antennae Galaxies (merger phase)

The Antennae galaxy was discovered on February 7, 1785 by William Herschel. It is 68 million light-years away and is in the constellation raven. After her English name " Antennae Galaxies " there are two galaxies. Taking its German name " Antennae Galaxies " are any indication, it is no longer about two galaxies, but only to a. In its original English name, the word " Antennae " translates as "feelers". It has the name because of their thin upturned tails (see remote view in the picture above left), which are reminiscent of sensors of insects.

The nuclei of the colliding galaxies have already penetrated here. The merger of the two galaxies in particular the interstellar gas is compressed and stimulated star formation. The star-forming regions thus formed and the associated emission nebula are visible as bright knots in the spiral arms.

The galaxy NGC 4038 (left ), was once a spiral galaxy and NGC 4039 (right) was a barred spiral. Before their first meeting, which took place before about 900 million years ago, the barred spiral NGC 4039 was (right) the larger galaxy. Meanwhile, it has become a smaller part of the system. 600 million years ago, the process was in today's stage of NGC 4676th The probe originated about 300 million years ago. In about 400 million years ago, a common stable core will have formed, as it exists today at the Starfish galaxy.

The Starfish Galaxy ( stabilization period )

The Starfish Galaxy (NGC 6240, IC 4625, UGC 10592 ) is about 400 million light years and is in the constellation Ophiuchus.

It is the result of collision and merger of two galaxies. The merger process is not yet complete and it has also been formed yet no regular structure such as ellipse or spiral. In the middle of Starfish galaxy two active supermassive black holes orbiting at a distance of 3,000 light- years of each other. They were the cores of the two galaxies output. The two black holes emit a large amount of X-rays. You will merge in a few hundred million years.

The Starfish galaxy was discovered on July 29, 2001 with the Chandra X-ray Observatory. She was the first galaxy in which they found two supermassive black holes in the core. This system is a prime example of a starburst galaxy, where a high rate of star formation there.

Ring galaxies

Occasionally leads to the formation of galaxies with ring structures. In ring galaxies a galaxy of another galaxy was pierced. By a compression wave generated as a current to the outside in the plane of the blue ring galaxy. Ring galaxies are unstable in contrast to polar ring galaxies.

The most well-known ring galaxy is the Wagenradgalaxie in the constellation Sculptor ( Sculptor ). With 500 million light years, it is relatively far away and no longer listed in the NGC catalog. The Ring of Wagenradgalaxie has a diameter of 150,000 light years. He is unstable and moves 340,000 km / h ( equivalent to 94 km / s) away from the center.

Interactive satellite galaxies

It is relatively common that a larger galaxy incorporated a dwarf galaxy. The time for this is from the first touch of spiral arms until full incorporation into the center about 900 million years. Such a first touch of the spiral arms can be found currently in the known Whirlpool galaxy and its satellite galaxy.

The Whirlpool Galaxy ( Messier 51 or NGC 5194/5195 ) in the constellation of hunting dogs ( Canes Venatici ) is about 31 million light- years away. She is well known spiral galaxy from the Hubble type Sc, that is, with more pronounced spiral structure. The interactive companion to the Whirlpool Galaxy is of irregular type. In NGC catalog he has the number NGC 5195 (M51 itself has the number NGC 5194 ).

In M51 currently an unusually active star formation takes place, which is probably 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 lifetime of only a few million to a few tens of millions of years.

The center of the whirlpool galaxy, the active galactic nucleus is strikingly hot. There stellar winds, expanding supernova remnants and the accretion of matter found in the central black hole. In addition, numerous collisions between gases take place.

Are also in the spiral arms of M51 active star formation regions and thus many young hot stars. The star-forming region in the spiral galaxy M51 and between the companion is particularly pronounced in this regard.

Research methods for interacting galaxies

The study of interacting galaxies falls into the area of ​​extragalactic astronomy. Here comes a wide range of astronomical methods used.

The most important part of the area called Astro spectroscopy. Astro spectroscopy is the wavelength-dependent analysis of the radiation of astronomical objects, including interacting galaxies and their components such as galaxy nuclei, gaseous nebulae, stars and supernovae belong. These analyzes are by wavelength divided in infrared astronomy, radio astronomy, ultraviolet astronomy, X-ray astronomy, gamma-ray astronomy and the range of visible light. It cooperates also with Multispektralanalysen and overexposure. By redshifts in radiation spectra differences in distance of galaxies can be determined. It is based on so-called astronomical standard candles.

In the spectrum of ultraviolet light can make particularly good high star formation rates. With radio astronomy, however, can best be studied active galactic nuclei, which are very strong in radio galaxies and Seyfert galaxies. Active galactic nuclei emit unusual from a lot of radio radiation. By galaxy mergers often generate large elliptical galaxies with active galactic nuclei (see Starfish galaxy) and the link between other interactions and the activities of galaxy centers represents ( cf. Whirlpool Galaxy ) an interesting object of study. On a radio map of an elliptical galaxy can be seen, for example, radio bubbles that are larger by a multiple than the optically visible part of the light.

However, if you in elliptical galaxies in the optical range a normal exposed shot with a strongly overexposed shot compares, then you also recognizes an entity that is greater many times over than the object at normal exposure: The gas in a galaxy merger by the star formation outbreak (see section "merger " phase ) is driven out of the system extends in a large area around the newly formed galaxy.

The two pictures on the right are photographs of the Spitzer telescope. Both images show the same view of about 300 million light -years distant galaxy group Stephan's Quintet in the constellation Pegasus. Stephan's Quintet is the most studied of all the compact galaxy groups. It has been able to discover exceptionally many special phenomena in this group. This unusual constellation is very turbulent and can thus draw conclusions about events that took place in the relatively young universe about 10 billion years ago.

For example, you can see the biggest shock wave in the upper receiver that has ever been observed. Four of the five galaxies are on a collision course. The incident quickly galaxy NGC 7318B (middle left of the image, double left eye) caused a 870 km / s fast shock wave that can be seen as a green train approximately in the center of the upper receiver. The wave front has a greater extent than our own galaxy.

In the upper recording several spectra are superimposed: X-ray, infrared and radio radiation to visible light was still mixed. The components of each serve to make certain things visible. To our eyes, the shock wave is actually invisible (see image below). The Spitzer telescope can detect infrared radiation from normally invisible objects such as dust particles or hydrogen molecules. The shaft along its movement stimulates hydrogen molecules to emit infra-red radiation, characterized the shape of the wave -cut.

Through the infrared radiations (green area shown ) can be calculated how fast the wave moves: In the spectroscopy can resolve the emissions of a gas, in this case hydrogen, in its spectral components. By manufacturing or moving, as is the case with the shock wave, move these components ( spectral lines ) depending on the relative velocity. These shifts in the spectral lines are called Doppler broadening. Since they are dependent on speed, they point to the relative speed of the shaft.