Galaxies are not distributed uniformly throughout space -- most are clumped together in clusters and superclusters that are evidently held together by gravity due to dark matter between the galaxies.
In fact, our own Milky Way galaxy belongs to a small cluster of galaxies, called The Local Group, which contains two large spiral galaxies (the Milky Way and M31), one medium-sized spiral (M33) and more than 30 other galaxies, most of them irregular galaxies much smaller than the Milky Way. For a complete listing and more details, see The Local Group of Galaxies.
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The most massive galaxies of the Local Group |
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Galaxy |
Distance (106 lt-years) |
Mass (109 Suns) |
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1000 |
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2.5 |
1500 |
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2.7 |
25 |
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0.16 |
20 |
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0.19 |
6 |
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The Local Group of galaxies. The figure (from Cyberia/Cosmos) is about 4 million light years on a side. |
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The Andromeda Galaxy M31 and its two satellites, M32 and M110, is almost a twin of the Milky Way. |
M33, the third largest galaxy in the local group. |
The Small Magellanic Cloud (SMC). The spherical object on the right is the globular cluster 47 Tucanae |
The Large and Small Magellanic Clouds (LMC and SMC) are named after the great explorer Ferdinand Magellan, who recorded them during his journey around the world in 1520. We can't say he discovered them because they are easily visible to the naked eye. Magellan called them "noctilucent clouds." Today we know that they orbit the Milky Way, and that they contain billions of young stars.
Within 30 million light years there are a few other groups of galaxies similar to the local group, listed here.
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The Coma Cluster of galaxies, at a distance of approximately 300 million light years, contains some 6,700 galaxies. |
The nearest giant cluster of galaxies is the Virgo Cluster of galaxies. It contains more than 2,000 galaxies, many of them larger than the Milky Way. The distance of the Virgo Cluster from the Milky Way is still somewhat uncertain, but a good estimate is about 50 million light years.
Astronomers have found thousands of clusters of galaxies. You can find images of several in Galaxy cluster mug shots from the University of Arizona.
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The violet overlay on this optical image of a cluster of galaxies represents X-ray emission from hot gas between the galaxies. Source. |
Astronomers have discovered a very important fact about the rich clusters of galaxies: they are powerful X-ray sources. The X-rays are emitted by hot (temperature of a few millions of degrees) gas that fills the space between the galaxies. Compare the optical and X-ray images of clusters in Galaxy cluster mug shots. The mass of the X-ray emitting gas in a rich cluster is comparable to the net mass in all the galaxies. But, as we shall see in the next section, the clusters are held together by an even greater mass of "dark matter".
Although about 80% of all large galaxies are spirals and 20% are ellipticals, the proportions are reversed in rich clusters. We know two reasons why this is so. First, the galaxies in rich clusters are all moving through the hot intergalactic gas in the cluster at speeds of hundreds of kilometers per second. This hot gas acts as a powerful wind blowing through the galaxy, which will remove any gas produced by stars and supernovae. This hot wind prevents the recycling of gas from stars to the disk of the galaxy that replenishes the interstellar medium of spiral galaxies. The second reason is that galaxies in rich clusters collide with each other, and galaxies that have suffered many collisions tend to become elliptical in shape.
Superclusters: Most galaxies do not reside in rich clusters like Virgo or Coma -- they reside in small groups like the local group. But these groups are not uniformly distributed throughout the universe. Instead, individual galaxies, small groups, and rich clusters are distributed in a kind of "foamy" texture consisting of huge sheets and filaments of galaxies separated by large "voids" where very few galaxies are found. The typical diameters of the voids are 50 - 100 million light years. You can see some of this texture below. But this map gives no information on the relative distances of these galaxies.
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All-sky projection of the nearest 30,000 galaxies, within a distance of about 1 billion light years. Few galaxies are seen in a band along the equator, called the zone of avoidance, where stars and dust in the Milky Way make it difficult to find galaxies. The Coma Cluster is at top of the figure. The band of galaxies extending from Coma to Hydra is the local supercluster, and that on the lower left is the Perseus-Pisces supercluster. |
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This image is very similar to the real distribution of matter in the universe - we see huge rich clusters, smaller clusters, little groups like our own Local Group, and big voids with no galaxies in between. But if you think that you are looking at the distribution of galaxies in the universe, click on the image! |
What astronomers really need is a 3-dimensional map of the universe that shows not only the locations of the galaxies in the sky, but also their distances. They can do that by taking spectra of thousands of galaxies and then inferring their distances from the redshifts of their spectral lines and Hubble's Law. Many groups of astronomers are conducting such redshift surveys. They are huge efforts. Up to now, only a few strips of the sky have been mapped in this way; but we can already see that they are telling us something very important about the texture of the universe.
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One of the most distant surveys of galaxies, the 2dF Redshift Survey of nearly 100,000 galaxies (click on the image to view full-size). In this depiction, the radial distance from the center (the Milky Way) indicates redshift determined from the spectra of the galaxies. According to Hubble's Law, the outer boundary (redshift z = 0.3) corresponds to a distance of about 2.5 billion light years. We see that the galaxies are not distributed uniformly, but have a "spongy" texture, with the superclusters surrounding "voids" having characteristic dimensions of about 100 million light years. Dark radial streaks occur in directions where the observations have not yet been made. |
For more details and images of about clusters and superclusters of galaxies, see http://zebu.uoregon.edu/timages/toc3.html
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The above movie (Source) shows the density of galaxies moving outwards in increasingly distant shells in redshift (shown on the counter). Dark colors show underdense regions, bright colors are overdense. |
Several other redshift surveys are underway to map the distribution of galaxies in the universe. Another important one is the Las Campanas Redshift Survey. The most ambitious one is the Sloan Digital Sky Survey, a new 2.5 meter telescope at Apache Point, New Mexico especially designed to observe more than 100 million galaxies and measure spectra of more than 1 million of them. Using this telescope full-time, the project will take about a decade to complete.
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Last modified March 20, 2002
Copyright by Richard McCray
