Distribution of Galaxies

> Distribution of Galaxies

Galaxies are generally not isolated in space but are usually members of small or moderate-sized groups, which in turn form large clusters and superclusters of galaxies. Our galaxy is one of a small group of about 40 galaxies that astronomers call the Local Group. The Milky Way and the Andromeda Galaxy are the two largest members, each with 100 to 200 billion stars. The Large and Small Magellanic Clouds are nearby satellite galaxies, but each is small and faint, with about 100 million stars.

Distribution of the Galaxies

This optical map, covering about 4,300 square degrees, or 10 per cent of the sky, shows the distribution in space of some 2 million galaxies. Galaxies tend to clump together—in this image, black represents areas of empty space and blue represents the galaxies. The image suggests that galaxies dot the surfaces of giant interconnected bubbles surrounding immense voids of empty space.

The nearest cluster of galaxies is the Virgo cluster; the Local Group is an outlying member of the cluster, which contains thousands of galaxies of many types. They all share a common direction of motion, the cause of which might be a supercluster hidden from view by our own galaxy, since superclusters up to 300 million light years across are known.

Hubble Deep Field

In December 1995 astronomers were able to prove that there are five times more galaxies in the universe than previously thought. Helping them in this conclusion was a composite image of photographs taken from the orbiting Hubble Space Telescope. Covering a tiny speck of the sky, the image, known as the Hubble Deep Field, depicts galaxies as faint as magnitude 30 (about 4 billion times fainter than can be seen with the naked eye). Formed some 12 billion years ago, they were the the most distant galaxies ever seen.

Overall, the distribution of clusters and superclusters of galaxies in the universe is not uniform. Instead, superclusters of tens of thousands of galaxies are arranged in long, stringy, lace-like filaments, arranged around large voids. The Great Wall, a galactic filament discovered in 1989, stretches across more than half a billion light years of space. Cosmologists conjecture that dark matter may exist in sufficient quantities to generate the gravitational fields responsible for the heterogeneous structure of the universe. In 1998 astronomers using the ROSAT orbiting X-ray observatory announced that the pattern of heat distribution in clouds of gas surrounding galaxy clusters demonstrated that the galaxies themselves must have formed before they began grouping into clusters and larger structures.

Virgo Cluster of Galaxies

The Virgo Cluster is a gravitationally bound group of galaxies lying some 50 million light years from us. It is the nearest galaxy cluster to the Local Group of galaxies in which our own Milky Way is situated, and it is moving away from the Local Group at several hundred kilometers per second. The two largest galaxies in this image of the centre of the cluster are the bright ellipticals M84, left, and M86, centre.

The most distant galaxies yet observed are those in the image known as the Hubble Deep Field, obtained with the Hubble Space Telescope in December 1995, and a follow-up image of a portion of the same area of sky imaged in the infrared in October 1998. The Deep Field contains galaxies as faint as magnitude 30 (some 4 billion times fainter than can be seen with the naked eye), which are estimated to have been formed about 12 billion years ago, when the universe was only about 5 per cent of its present age. Extrapolated to the whole sky, the image implies a five-fold increase in the number of galaxies over previous estimates. The infrared image in 1998 was able to capture even fainter, cooler galaxies. The target location, in the constellation Ursa Major, was chosen because it contained no major foreground objects that would obscure the view of distant galaxies.

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