Hydra Cluster isn't the largest galaxy

Of clusters and superclusters

A look into the future of the universe

In the current issue of New Scientist you can gain an insight into the future form of space.

Stuart Clark, visiting professor at the University of Hertfordshire gives an overview of the state of research on clusters (star or galaxy clusters) and an idea of ​​the possible future of the universe from superclusters. Clusters of galaxies are the largest known formations in the universe. They are so huge that galaxies appear to be tiny spots inside them, they encompass hundreds or thousands of galaxies in a cloud of gas.

New images (x-rays) from NASA's Chandra missions and ESA's XMM mission show that the clusters are in turmoil. Chandra and XMM's instruments analyze the temperatures in the gas with exceptional accuracy. The only logical explanation for the great unrest is that the galaxy clusters collide. And when they crash into each other, they become new gigantic units and they eat the matter around them at the same time. When this process is completed one day, astronomers believe, the universe will be a place full of huge superclusters.

Galaxies and clusters

So far, clusters have been found sporadically, just 10% of all galaxies exist as part of these formations. For the 15 billion years since the Big Bang, galaxies have been the basic building blocks of the universe. With a hundred billion stars in a multi-armed spiral shape with a diameter of 50,000-100,000 light years, the Milky Way is a lightweight among the galaxies. "The average structure we expect today is a group of four or five galaxies," explains Frazer Pearce of the University of Nottingham. Our galaxy also belongs to a loose galactic union dominated by the Andromeda galaxy and the Milky Way.

Computer simulations

Using computer models, scientists have tried to uncover the secret of the origin of the universe and its further development. It has been known for a number of years that space has been expanding at increasing speed since the Big Bang. The reason for this acceleration is still unknown, but the value of the Hubble constant, i.e. the number that determines the speed at which the galaxies have moved away from each other since the Big Bang, has been calculated (see How quickly the universe expands ?). The expansion and movements of space are entered into appropriate programs and extrapolated. Pearce has performed such simulations and is certain that galaxy clusters continue to grow by swallowing up smaller neighboring galaxies: "In the simulations you can clearly see small knots of matter everywhere, from the size of galaxies to small groups of galaxies that are in the clusters flow in. And we think that this is exactly what happens in the real universe. "

With normal telescopes, which are limited to the visible wavelengths of light, one can only see the galaxies themselves. Even if two galaxies collide, it looks no different than if they exist peacefully. It is only when you look at the extremely hot gas that surrounds you that it becomes clear what is going on. The temperatures in this gas are between 50-100 million Kelvin and they emit X-rays. X-rays such as those taken by Chandra or XMM therefore provide the decisive clues as to what is going on in and around the clusters. The different temperatures are made visible and maps can be created showing the distributions. "These cards turned out to be much more chaotic than expected," comments Benjamin Mathiesen of Stanford University in California

Intergalactic cold fronts

Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics paid particular attention when analyzing Chandra images for shock waves in the gas, which are indicative of collisions. He looked for temperature jumps in the gas and found the opposite: areas where the temperature suddenly dropped rapidly. At first he was disappointed, but then he realized that he had found the remains of the galaxy's last meal, the part of a smaller cluster that had escaped absorption for the time being. He called these areas "intergalactic cold fronts". At the second attempt he also found the heat front for it: "In at least 2 clusters with cold fronts, we saw weaker fronts at some distance, which must be the genuine shock fronts."

Cannibalism in space

The extent of the fusion processes amazed the astronomers. About half of the galaxy clusters examined had just fed, and that to a considerable extent: perhaps a few dozen galaxies each. Overall, it is estimated that 10% of the clusters in the universe are currently in mergers. A merging process takes millions of years, but in the process considerable energy is released, which makes it clear that something is in progress.

It very much looks like the increasing clustering and enlargement is an unstoppable process. In a few tens of billions of years there will be the age of much more massive structures. Mathiesen: "The next step will be superclusters."

Our own loose group of galaxies will be drawn towards the Virgo (Virgo) cluster and may merge with it in a few tens of billions of years. However, the chances of our delicate spiral galaxy surviving this unscathed are not good. The analysis shows that spiral galaxies are usually compressed to form elliptical galaxies. Often there is an elliptical galaxy in the middle of a cluster, a monster that has grown mightily from eating its neighbors. In the Virgo this is the galaxy M87. And there is a monster within the monster, because M87 also contains a black hole (cf. voracious beasts in space), three billion times more massive than our sun and probably the same in other central galaxies. As the clusters continue to grow, these black holes will also develop into giant beasts. Our Milky Way will probably have its arms torn off in the distant future and then be compressed into an ellipse - at least the part of our galaxy that was not swallowed up by the gigantic black hole. Not a nice idea.

Perhaps cannibalism is one of the primal mechanisms of the universe. It has only recently been proven that larger galaxies incorporate smaller brothers (cf. Galactic cannibalism?). If the astronomers are right, the greatest unit will always win in the galactic struggle for survival. Ultimately, however, the superclusters are driven apart by the constant expansion and then remain stable because they are too far apart for the gravity of another cluster to be able to pull them into it. (Andrea Naica-Loebell)

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