Super-galaxy
Clusters of Galaxy Clusters – Stellar Frontiers
Our galaxy, the Milky Way, is basically a large cluster of a few hundred billion stars lying in a disk-shaped region of space (typical of spiral galaxies) and containing many clusters and associations of stars.
Recent investigations into the phenomenon of clustering show that it does not occur on any preferred basis. Instead, clustering occurs on all scales, from a single galaxy to possible the entire observable universe. At the other extreme – that of particles such as atoms – clustering also seems to occur. Protons might be clusters of still smaller particles (various types of quarks). In fact, we might even think of our solar system as a cluster containing one star, nine planets, several dozen moons, and plentiful smaller bodies.The clustering phenomenon continues when we examine nearby galaxies: They form groups whose members are associated with one another. Our Milky Way is located in such a group called, simply, the Local group. The best counts indicate that the Local group contains about 27 member galaxies. Many of these are small, inconspicuous objects. But the Local group also includes some of the better known galaxies, such as M-31 in Andromeda, M-33 in Triangulum, and the Magellanic Clouds of the southern clouds.
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| Magellanic clouds and super-galaxy. Magellanic clouds. The largest nearby high velocity cloud, called the Magellanic stream, forms a long tongue of gas many hundreds of thousands of light-years in extent that links the Milky Way to the Magellanic clouds. It is possible that the Magellanic clouds and the gas stream, with the clumpiness seen within it, all formed billions of years ago from a single cloud of matter. The Magellanic cloud grew dense enough to form stars while the rest of the gas continued to swirl about in space. If this picture is correct, these are the first observations of extended clouds of intergalactic matter. Credit of the photograph: ESO/S. Brunier |
By definition, a group of galaxies (the Local group is a good example) is a clump of less than 10 to several dozen galaxies. A group that exists on a larger scale, with more members, is known as a cluster of galaxies. Clusters may contain up to 1,000 galaxies, all apparently born many millions of years ago from one gigantic cloud of gas. Members of these aggregations of galaxies seem to be bound together by gravitational force.
It has been suggested that the groups and clusters of galaxies closest to our own may combine to form sort of a super-galaxy. Astronomers have deduced the super-galaxy’s existence by examining the distribution of some of the nearer collections of galaxies (those within 50 million light-years from us). We find that that they lie predominantly along the equator of a hypothetical super-galaxy, just as the nearby stars in our nearby galaxy lie along the equator.
While not all the astronomers agree that the super-galaxy exists, there is some evidence (although circumstantial) attesting to its reality.
There is also evidence that distant clusters of galaxies also are collected in groups similar to the local super-galaxy. We might, therefore, imagine a whole universe full of super-galaxies. There is no way at present of telling whether or not there is organization on an even bigger scale.
Astronomers visualize the local super-galaxy as a flattened system with an estimated diameter of 100 to 200 million light-years and a thickness of about 20 million light-years. According to one estimate, our galaxy must move about the nominal center of this super-galaxy at approximately 300 miles per second, taking 100 billion years for a single round trip. Since this is a time much greater than the age of the entire universe, it’s hard to see how a super-galaxy could even have been formed and set in motion in the first place. Such motion, however, is extremely difficult to detect and might not be real.
The evidence that such a super-galaxy is a flattened system is somewhat similar to the appearance of a clear night sky that indicates we live in a flattened disk of stars called the Milky Way. We see our galaxy as a band of stars because we look along the disk of the galaxy into the long dimension. As a result we see many more stars than we do in directions at right angles to the galactic equator. A similar effect occurs when we look at distant galaxies. The apparent evidence that they, too, lie in a band (highly inclined to the Milky Way) is analogous to the distribution of stars in the day. The band of stars seems to to indicate the existence of a similarly flattened super-galaxy whose member objects are entire galaxies, not stars.
The center of the super-galaxy is believed to lie in the direction of the constellation Virgo. A huge cluster of galaxies containing perhaps 1,000 members is located here. It’s among the largest of the tens of thousands of galaxy clusters known. The total mass of the super-galaxy may be as great as a quadrillion 10 (15) star masses.
We can best appreciate the enormous scale of the super-galaxy by comparing it with that of our own galaxy and the surrounding area. The size of a typical star cluster in the Milky Way ranges from a few dozen to a few hundred light-years across: the galaxy itself is about 100,000 light-years in diameter. The Magellanic Clouds, our nearest neighbor galaxies are about 160,000 light-years away while M-31 is some two million light-years distant. The diameter of our Local group of galaxies is five to six million light-years. About 15 other groups of galaxies lie within 30 million light-years, another 40 groups or clusters are between 30 and million 50 million light-years away. These clusters lie roughly along the super-galactic equator, although there are of course exceptions. These groups of galaxies may have formed within an enormous primeval cloud of gas 15 billion years ago, much as star clusters formed in the spiral arms of our galaxy.
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| Pisces cluster, super-galaxy. An idea of the enormous scale of the universe can be had from this picture of a cluster of galaxies in Pisces, lying at the limit of the observable universe. At this distance, the Andromeda galaxy (M-31) would look no brighter than the brightest member of the Pisces cluster. Whether such clusters as these belong to a huge super-galaxy has yet to be determined. Hale Observatory photograph. Source de l’image : Astronomy Magazine |
Remarkably, the majority of galaxies within the super-galaxy – including all common spiral galaxies – are themselves flattened, and generally the direction of their flattening seems to be the same as that of the super-galaxy. Perhaps some process involving rotation of the primeval gas cloud controlled the formation of the individual galaxies
How could the clusters or groups of galaxies that comprise the super-galaxy arise in the first place? If we accept the theory that most matter in the universe formed in the big bang, we must then give some thought to the chain of events that took place as the universe expanded. There must inevitable have been some slight irregularities within the universe as it expanded and cooled, and the pull between these random irregularities held some of them in regions that became separated from each other. In other words, clumps formed, which were later to be the birthplaces of the galaxy clusters. In turn, and on a smaller scale within the proto-galaxy cluster clouds, there were additional sub-units that became gravitationally tied together that formed the proto-galaxies themselves. These individual clouds eventually broke into sub-units that became clusters of stars.
Thus, according to this theory, we find a whole hierarchy of condensations occurring in the universe as far down as the formation of stars, with their accompanying planets and moons. But while our theories of star and planet formation seem to be fairly sound, the formation of the galaxies themselves is a much more complex and difficult problem to unravel – and the formation of clusters of galaxies, or the super-galaxy, is almost a complete mystery: we can only make reasonably intelligent guesses.
It’s tempting to draw analogies between the formation of various objects in their hierarchy, but such analogies tend to be misleading since they ignore essential differences between the vastly different scales of time and size.
Studies of unusual clouds of hydrogen gas that appear to be approaching our galaxy have provided totally unexpected new evidence supporting the existence of the local super-galaxy. Scientists have proposed several different explanations for these clouds (called high velocity clouds), including the suggestions that matter may be falling into the Milky Way from intergalactic space, or that matter may drift between the galaxies at distances similar to those of the Magellanic Clouds or M-31. Alternatively, the gas may be part of the spiral structure of our galaxy – that lying most distant from the nucleus. The high velocity clouds, however, apparently lie primarily near the equator of the super-galaxy. The may be stretched along it! Perhaps the clouds float between the galaxies – some of them between members of the Local Group and others between the components of more distant groups of galaxies. Possibly they represent the matter remaining after galaxy formation – matter that never was able to begin the process of star formation. Somewhat similar clouds of hydrogen have been detected near the galaxies M-31, M-33 and M-81, and the clouds are apparently directly associated with these galaxies.
Don’t try to find the local super-galaxy at your next observing station – it’s a phenomenon that is impossible to observe visually. Super-clustering is, after all, one of the most esoteric properties of the universe so far discovered by astronomers. If you want to imagine further groupings beyond that mentioned here, feel free. For example, might we speculate on groups of universes? How far can we take the hierarchical picture down into the microcosm? Such questions must today remain unanswered.
(Astronomy, August 1976, by Gerrit L. Verschuur)
