Online Astronomy eText: Galaxies and the Universe
Clusters of Galaxies
A portion of the nearest cluster of galaxies, the Virgo cluster of galaxies, which is only about 50 million light years away. Near the center of the cluster is the giant elliptical galaxy M87 (on the left). Elliptical galaxies such as M87 consist only of a roughly elliptical (in this case, nearly spherical) cluster of millions, billions, or in this case about 3 trillion stars, moving in essentially randomly oriented orbits, as opposed to the circular motions of stars around the disk of our spiral galaxy. The bright portion of M87 shown here is over 120,000 light years across, considerably larger than the disk of our galaxy, and since it is spherical instead of a flattened disk, contains tens of times more stars and mass than our own galaxy. M87 is also unique in the number of globular clusters (the small specks clustered around it) which swarm around the galaxy. Whereas our own galaxy has only a hundred or two globular clusters, M87 is thought to hold sway over more than ten thousand such clusters. A supermassive black hole in the center of M87 is responsible for a jet, 6500 light years long, of superheated gases ejected from the center of the galaxy (not visible in this image). As large as M87 appears here, long-exposure images show fainter outer regions extending three to four times further into space, which despite their relative faintness, may contain as many or more stars than the brighter inner regions, because of their larger volume. Because of the large mass of the Virgo cluster, the galaxies in it have motions relative to each other of as much as 1000 miles per second, and since the cluster is close to us, and has a very small Hubble redshift, many of the galaxies in the cluster are actually approaching us, some with speeds of as much as 300 miles per second. (Robert Gendler, apod060520)

A wider view of the Virgo Cluster (M87, and all the other galaxies in the previous image, occupy only the central 10 or 20% of this image). The cluster spans more than five degrees, and contains more than a hundred galaxies of many types, several of which are among the brightest galaxies visible from Earth, because of the closeness of the cluster. The large mass of the cluster, which includes not only the visible galaxies, but also large amounts of thin, hot gas visible only at X-ray wavelengths, and even larger amounts of "dark" matter, is pulling our galaxy and the other galaxies in our Local Group of galaxies toward the Virgo Cluster. Thus, in a sense, we may be thought of as an outlying part of the Virgo "supercluster". (Digitized Sky Survey, Palomar Observatory, STScI, apod000220)

Another cluster of galaxies, Abell 1185 spans a million light years, and lies 400 million light years away. Most of the hundreds of galaxies in Abell 1185 are elliptical galaxies, but other types are also common, and because the cluster is so tightly packed, several galaxies are visible, such as the pair on the left, which are distorted by collisions with each other. (Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT, apod051122; Copyright CFHT)

(Click on the image above for a larger version)
A far richer cluster of galaxies two billion light years away, Abell 1689 is one of the most massive "objects" in the known Universe. The huge mass of the cluster so thoroughly warps the fabric of space-time in its vicinity that here and there, circular arcs can be seen, which are images of more distant galaxies, distorted by the passage of their light through the gravitational field of the cluster. However, it is not the mass of the visible galaxies, immense though it is, that bends the fabric of space in this way. Based on the strength of the "lens" created by the cluster, the mass of the cluster must be a hundred times greater than that of the visible stars. This result, extreme though it seems, is fairly typical for galaxies and clusters of galaxies. Every study of the mass of galaxies and clusters of galaxies yields estimates of the gravitational force that are ten to twenty times greater than the estimated mass of the visible stars (see Dark Matter In Galaxies for an introduction to this topic). The image below uses computer calculations of the lensing power of the cluster (based on the arcs and distortions of the galaxies seen all around it) to show the distribution and amount of dark matter in the cluster. That distribution, shown in blue, is most concentrated where the galaxies are concentrated. This is an almost universal result. Wherever there is more visible matter, there is more dark matter. Save for very unusual circumstances, the two always go together. Why that is, is completely unknown. Someday, we will know the answer; and once we do, it will probably be (or at least seem) so simple and obvious that we will wonder why we didn't realize it in the first place (save for the fact that in the absence of any way of observing dark matter, it is far easier to speculate about it than to solve the problem, and speculation tends to muddy the waters which good data clarify). (Credits for images above and below, NASA, ESA, E. Jullo (Jet Propulsion Laboratory), P. Natarajan (Yale University), and J.-P. Kneib (Laboratoire d'Astrophysique de Marseille, CNRS, France), Acknowledgment: H. Ford and N. Benitez (Johns Hopkins University), and T. Broadhurst (Tel Aviv University))

Below, three views of a gravitational lens created by a cluster of galaxies
(K. Sharon (Tel Aviv U.) and E. Ofek (Caltech), ESA, NASA, apod060524)
Seven billion light years away, in the direction of Leo Minor, is the large cluster of galaxies shown immediately above, SDSS J1004+4112 (the numbers are the spherical coordinates of the cluster). Near the center of the cluster are five bright white points which are actually multiple images of a single object, a distant quasar, created by a bending of the space-time through which the light of the quasar is passing, by the immense mass of the cluster of galaxies which lies in its path.
A closer view of the central portions of the cluster reveals that not only the quasar, but other distant galaxies have their light distorted, as it passes through the cluster. Arcs and rings of oddly shaped galaxies are in many cases, simply multiple images of such galaxies.
This full-scale view of the Hubble Space Telescope image even shows the faint glow of the host galaxy whose brilliant core is the quasar which is being imaged by the cluster of galaxies.

Abell 2667, a rich cluster of galaxies about 2.7 billion light years distant. The Hubble Space Telescope image above shows a substantial portion of the cluster. In the image immediately below, the left portion is shown in more detail. Near top left, a the spiral galaxy is apparently being torn to pieces (down and to the left of the galaxy), as it moves through the cluster. In reality, the stars in the galaxy are more unaffected by its passage through the cluster, than not. But there are huge amounts of gas lying between the cluster galaxies, due to past collisions of those galaxies; and as the spiral moves through the gas, its own gas is stripped away and compressed, causing knots of hot, bright stars to form in the region behind it. (NASA, ESA, Jean-Paul Kneib (Laboratoire d’Astrophysique de Marseille) et al., apod070305)
In the image below (the right portion of the first image), we see even more distorted "galaxies". In this case, however, these are optical illusions caused by the passage of light from far more distant galaxies, through the "gravitational lens" of the intervening cluster.