Clusters of Galaxies

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Galaxies love to cluster together! Their mutual gravity can draw galaxies together into a cluster that is several millions of light years across. Some clusters have only a handful of galaxies and are called poor clusters. Other clusters with hundreds to thousands of galaxies are called rich clusters. The low mass of a poor cluster prevents the cluster from holding onto its members tightly. The poor cluster tends to be a bit more irregular in shape than a rich cluster.

Our Milky Way is part of a poor cluster called the Local Group (an imaginative name, yes?). The Local Group has two large spirals, one small spiral, two ellipticals, 13 irregulars, and 14 dwarf ellipticals. There may be more irregular and dwarf ellipticals. The distribution of the galaxies is shown in the figure below. The Local Group is about 3 million light years across with the two large spirals, the Milky Way and Andromeda Galaxy, dominating the two ends. Each large spiral has several smaller galaxies orbiting them. The proportions of the different types of galaxies in the Local Group probably represents the number of the different types of galaxies in the rest of the universe. The small galaxies can be seen in the Local Group because they are close enough to us. But the dwarf galaxies are hard to see in far away clusters.

the Local Group

The closest large cluster to us is the moderately-rich cluster called the Virgo Cluster because it is in the direction of the Virgo constellation. It has many hundreds of galaxies (mostly spirals and irregulars) distributed into an irregular shape about 10 million light years across. It is about 49 to 59 million light years from us. Some ellipticals are present in the central part of the cluster including a giant elliptical at the center (M87) that has become so large by gobbling up nearby galaxies that were attracted by its enormous gravity. The total mass of the Virgo cluster is large enough that its gravity pulls nearby groups of galaxies (including the Local Group) toward it.

Virgo Cluster center
The center of the Virgo cluster has three giant ellipticals, M 87 (bottom left corner) and M 84, M 86 (center right). M 86 may be a lenticular S0 type instead.
Courtesy of AAO

A rich cluster is seen in the Coma constellation---the Coma Cluster. It has thousands of galaxies (mostly ellipticals and S0 spirals) in a large, spherically-shaped cluster about 300 million light years from us. It is at least ten million light years across. The elliptical galaxies congregate toward the central regions while the few spirals are found on the outskirts. Two giant ellipticals (NGC 4874 & NGC 4889) occupy the central part of the cluster. Like M87 in the Virgo Cluster, they have grown very large from pulling in galaxies that were unfortunate to have strayed too close to escape the giant ellipticals' gravity. Other rich clusters show the same segregation of the spirals from the ellipticals as the Coma Cluster. Ellipticals gather together in the center of clusters while spirals prefer to stay close to the periphery.

Coma cluster
The Coma cluster is a rich galaxy cluster with thousands of galaxies.
Courtesy of NOAO/AURA/NSF
Hercules cluster
The Hercules cluster is a poor cluster with less than a hundred galaxies.
Courtesy of NOAO/AURA/NSF


The clustering phenomenon does not stop with galaxies. Galaxy clusters attract each other to produce superclusters of tens to hundreds of clusters. Their mutual gravity binds them together into long filaments (thin, stringlike structures) 300 to 900 million light years long, 150 to 300 million light years wide, and 15 to 30 million light years thick on average. The discovery of these huge structures was made recently from years of taking doppler shifts of thousands of galaxies. The doppler shifts of the galaxies were converted to distances using the Hubble Law. Between the filamentary superclusters are HUGE voids with very few (if any) galaxies. The voids are typically 150 million light years across.

Two pioneers in the mapping of the structure of the universe are Margaret Geller and John Huchra. They and their students took thousands of spectra of galaxies along thin pie-shaped slices of the sky over 15 years to produce the map with 2 slices extending out about 400 million light years shown below. It would take much too long to take spectra of galaxies in every direction in space, so astronomers map the universe in slices.

slices of the universe
Courtesy of Geller, da Costa, Huchra and Falco.

In 2003, the Anglo-Australian Observatory released a much larger survey (``2dF Galaxy Redshift Survey'') of over 221,000 galaxies in two slices that extend over 1.5 billion light years in a two-degree field of the sky. The 2dF survey system could take the spectra of 400 objects simultaneously so it took them ``only'' 5 years to complete the survey. It is shown in the figure below. Each blue dot is a galaxy. The Sloan Digital Sky Survey will greatly expand the volume to a million galaxies in one-quarter of the entire sky. It is expected to be finished in the second half of 2005.

2dF survey slices

The arrangement of the superclusters and voids looks like a bunch of soap bubbles or swiss cheese with the galaxies on the borders of the huge holes. Although the picture above is only a two-dimensional version of the three-dimensional map, you can still see the lacy, foamy structure. What produces the long thin strands of clusters around the huge bubbles of empty space? Obviously, gravity is the force at work, but how has it worked to produce these structures? Dark matter must play a significant role but how it does that is not known. Astronomers are using powerful supercomputers to simulate the gravitational interactions of millions of particles and they program guesses of the behavior of dark matter into the simulation code. Early results from the simulations are able to produce the filamentary structure and voids.

The picture below shows a supercomputer simulation of a 300 million light year cube of the universe using 47 million particles interacting with each other through the force of gravity. The galaxies clump together into clusters and the galaxy clusters gather together into huge string-like superclusters with big gaps (voids) in between. The colors represent differences in density: from lowest to highest density the colors are black, blue, pink, red, orange, yellow, and white (highest density peaks). The image is a much compressed jpeg version of a gif image from the HPCC group at the University of Washington. Some details have been lost in the compression, but you can select the image to bring up the original uncompressed gif file in another window.

simulation of large-scale structure growth

Another nice set of pictures and movies of the formation of the large-scale structure can be found at:

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last updated: June 3, 2004

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel