We have seen that, as they evolve, galaxies tend to merge and enlarge. Does this means that, now in the present day, we only observe isolated galaxies in the sky ?
As we have seen before, galaxies are born inside huge matter clouds. As a result of this, galaxies have a tendency to congregate in clusters, in the same way as stars which are often born in groups.
A view of the Coma cluster, that lies 100 Mpc away.
It contains more than 1000 bright galaxies.
At the center of such a cluster, there often lies one or more giant
elliptical galaxies, results of frequent collisions.
As we go away from the center, we can find more spiral galaxies.
Between the galaxies of a cluster, there is a cloud of hot invisible gas, that can be detected at X-ray wavelenghts.
Inside a cluster, galaxies move, being attracted to each other by the gravity.
Abell 1060 cluster in Hydra
field of view : 1Mpc
at optical wavelenghts
|in the X-ray part of the spectrum|
In our neighbourhood, our galaxy, the Milky Way, belongs to the Local Group, a small elliptical cluster, about 7 million light years wide. It contains about 30 galaxies, the two most important are the Milky Way and the Andromeda galaxy (M31), and belongs to the local supercluster, called the Virgo supercluster.
|The distribution of the nearest clusters inside the Virgo supercluster.
At the center of the local supercluster, we find the Virgo cluster. Lying about twenty Megaparsecs away, it seems to attract the local group, with its tremendous mass. At the center of the Virgo cluster, there lies M84, M86 and M87, three giant elliptical galaxies, twenty times larger than our own galaxy.
The clusters of galaxies are themselves gathered in superclusters, inside which they are attracted to each other by gravity.
A supercluster is stretched out into a kind of giant wall, 300
to 900 million light years long, 100
to 150 million wide and 15 to 30 million thick.
These walls are separated by voids, where there are very few galaxies. Such a void is typically about 150 million light years wide.
The universe within 1 billion light years.
You can clearly perceive the structure in walls of superclusters and the voids.
Source : Richard Powell
On the upper slice, we can distinguish the Great Wall (highlighted in red), which stands in the space at mid-height.
This filamentary structure seems to be the fundamental framework of the universe. Computer simulations have shown that particles interacting by gravity will conglomerate into this shape.
Source : University of Washington
Does the fact that we observe this structure in walls means that
our universe has reached a final stage of evolution ?
We shall still need a lot of observations with more and more powerful telescopes in order to be sure of such an assertion.