Galaxy groups

Structure of galaxy groups

Galaxy groups are the biggest structures of the Universe. They are constituted by hundreds of galaxies bound together by their own gravitational attraction. Between the galaxies we find some matter constituted by warm gas there, forming a plasma, the temperature of which reaches 10 to 100 million degrees. This plasma is a strong transmitter of X-rays. The spectrum analysis of these X-rays informs about the temperature of the heap and about its dynamics. Galaxy groups form in the crossings of the strands which compose this cosmic cobweb. The Universe being constituted by matter distributed along its immense strands between which are vast bubbles empty of matter.

The matter passes by along strands and converges on the knots of the cloth to form groups, then galaxy groups. Galaxy groups are not constituted that of galaxies, they are filled with extremely warm gas (10-100 million degrees) and of weak density (1000 particle / m3). This gas is distributed in a much more diffuse and more vast way than the galaxies. These gases fill the space between the galaxies and constitute the essential of the groups, representing a mass more important than the galaxies themselves. In these temperatures, it is totally ionized, it is about a plasma.

Structure of galaxy groups

The local group

The Milky Way and the Big Nebula of Andromeda have satellite galaxies. These two huge spirals belong to an even bigger grouping of 20 galaxies among which M31, M33, Maffei I and Maffei II, the Big and the Small Cloud of Magellan, conscript Groups it local. All these galaxies move around a common center situated between our Galaxy and the Nebula of Andromeda. All the observers know the biggest and the most brilliant of all: M31, in the constellation of Andromeda.
The most known heap of galaxies are Virgo, Pegasus I, Pisces, Cancer, Perseus,

Coma, Ursa Major III, Hercules, Pegasus, Heap A, Centaurus, Ursa Major I, Leo, Gemini, Corona Borealis, Heap B, Bootes, Ursa Major II, Hydra II Distant from 2,5 million light years, it is the closest galaxy but especially it is the only one who is visible in the bare eye, on dark night.
We can consider the Local Group, or the other groups looking like it as heap containing a number relatively restricted by objects.
Galaxy groups can be more important. If we come down until the magnitude 21, there would so be 75 million galaxies among which our.

The local group is a part of an enormous complex of 10000 galaxies assembled in heap extending over some 200 million years lights, conscript local super clusters or super clusters of the Virgo.
The super clusters of the Virgo and super clusters of the Hydra and the centaur fall themselves towards another big conglomeration of galaxy groups which we call the Big Attractor. Galaxy comes from Greek " galactos " which wants to say milk.

A magnificent ballet

Of our ground base, we participate in a fantastic cosmic ballet: the Earth propels us in 30 km/s around the Sun which splits the space in 230 km/s around the Milky Way.

This one falls in his turn towards the galaxy of Andromeda in km/s 90, each of its galaxies rush in 45 km/s towards the center of the local Group, our galaxies group.

The local Group moves in 600 km/s attracted by the galaxies group of the Virgo and the super clusters of the Hydra and the Centaur, who falls in his turn towards the big Attractor.

The heap of the Virgo ( Virgo) is a massive group of galaxies which dominates the super clusters of the Virgo. It is the closest heap of galaxies of our Galaxy, the Milky Way. There are approximately 2000 galaxies in this heap (but 90 % of them are dwarfish galaxies). This heap is one diameter of about 15 million years light, hardly more than our Local Group, but it contains fifty times more galaxies.

The group of galaxies Virgo, dominated by M86 (at the top), M84 (to the right), NGC 4388 below and NGC 4387 in the middle.
Source: Hawaiian Starlight, CFHT

The heap of Coma is a galaxies group, spherical, very dense in its center. It contains more than thousand galaxies and is situated, in 300 million light years with the heap Virgo in the constellation of the Virgo.

The super clusters of Coma (Bernice's Hair) is one of galaxies group the most known.

The group of galaxies Fornax is in about 65 million light years of the Earth. The galaxies of the heart of the heap Fornax seem to move in the direction of a common point, attracted by the dominant gravity of invisible structures of dark matter of this region.
There is in the direction where is situated the System of the Furnace a dwarfish galaxy, distant from 600 000 light years. It belongs to the Local Group, that is to the small heap of galaxies in which is also the Milky Way, our galaxy.

The group of galaxies Fornax of the look-out observatory Chandra reveals emissions with high energy of several huge galaxies close to the center of the heap Fornax and an immense diffuse cloud of warm gas emitting X-rays.

On the photo above, the blue objects are galaxies and the yellow objects are stars in the foreground. In the center of the group, we find a gigantic elliptic galaxy (ESO444-46) of a diameter of more than 340 000 light years.

The group of galaxies of the Hydra, nicknamed so because of the constellation which accommodates it, covers approximately 10 million light years and contains more than 100 brilliant galaxies. The study of the radiation X emitted by the gas which it contains in quantity between its galaxies revealed an abnormally important proportion of dark matter. Abell 1060 ( A1060) is the other name given to this group hydra. With the group of the Virgo and that of the Centaur (A3526), the group of the Hydra is one three bigger group of galaxies unless 200 million light years of the Milky Way. It is current to refer to this region of the sky under the name of ' super clusters Virgo -Hydra - Centaurus '.

Stephan's Quintet is a group of galaxies, that is a group of very close galaxies some of the others. It contains normally 5 main galaxies but only 4 are visible on this image. The galaxies incur because of their strong mass and it is likely that they will merge one day. The galaxies are strongly perturbed by the effect of their gravitational interaction. We notice it on this image by the distended forms of the strands which extend very far from the center of the galaxy.
The members of this group are NGC 7317, NGC 7318a, NGC 7318b, NGC 7319 and NGC 7320. If the first 4 galaxies form a relatively compact group, the fifth, NGC 7320 is away from the group but situated in the same region.

The group of galaxies of Stefan's Quintet. Well known by the amateurs, the group of the Quintet is situated in the constellation of Pegasus, about a distance about 340 million light years of the Milky Way and was discovered in 1877 by the French astronomer Edouard Stephan, since the look-out observatory of Marseille. Credit NASA / ESA

The distribution of the galaxies is not also homogeneous as it. They group together inside long concentrations, called strands, and leave between these strands of immense deserted spaces, the big space. Strands, which represent only 10 % of the space, so constitute a network of knots, a sort of three-dimensional cobweb. In the intersection of these knots we find smaller groupings: regions of meeting particularly rich in galaxies, which represent super clusters of galaxies. In super clusters, the galaxies can again group together in heap smaller which are clouds and heap of galaxies.

In this gas of galaxies, there is neither concentration in certain places, nor no privileged direction, each lives his life there, moving in its speed in a some direction of the space. This model which describes the universe in general is known under the name of cosmological principle. The universe seems to very very homogeneous large scale, but its weft becomes visible on scales included between 100 and 300 million light years. Its structure represents itself under the shape of strands, pages or pancakes. This concentration of galaxies has a structure in sponge. Here is the way we can evoke the cosmic weft of the universe.

The average distance between two galaxies in a heap is 5 million light years. It corresponds to 200 times the average diameter of a galaxy. Between two stars inside a galaxy, the distance is worth tens of million times their diameter. Even in the most diluted of heap, the galaxies are much more moved closer between them than are it, proportionally, stars inside a galaxy. Contrary to what takes place with the stars which have practically no chance to meet, the galaxies can very well collide the some with the others. The links are such as the gravitational effects (effects of tides) inferred by their nearness has an influence on their morphology.

The group of Quintuplet is situated unless hundred light years of the galactic, old center about four million years. We see one of the most brilliant stars of the Galaxy the star of the Pistol there, the name of which explains by the shape of the small nebula in which it is immersed.

M80 is a beautiful spherical heap of 8th magnitude is one diameter 86 light years. Visually it looks like completely a comet. This very supplied stellar swarm contains several hundreds of thousand stars, maintained together by the gravitational strengths. It is one of heap the densest of our Milky Way. M80 is one of Charles Messier's personal discoveries. William Herschel described it as one of the richest heap of stars.

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Galaxy groups

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Astronomy - october 15th 2007