SP-466 The Star Splitters



Clusters of Galaxies


[151] One of the primary goals of the HEAO project was to open wide the high energy window to the realm of the galaxies. This goal has been achieved. Galaxies of all sizes, shapes, and types have been detected. Some are relatively quiet, whereas others are exploding and ejecting jets of high energy particles that have been observed with radio, optical, and X-ray telescopes. Some galaxies evolve in isolation; others are found in clusters of galaxies, where they are constantly interacting with other galaxies in ways that profoundly affect their structure and evolution. Often they are stripped of the supply of gas needed to form new stars; sometimes they are gobbled up by other galaxies that are called, appropriately, cannibalistic galaxies.

Soon after it was realized that Andromeda and other spiral nebulae are galaxies of hundreds of billions of stars, it was recognized that galaxies are not distributed at random over the sky, but clump together in groups and clusters. About 10 percent of all galaxies are members of rich clusters of thousands of galaxies. One of the important discoveries of the high energy satellite experiments of the 1970s was that rich clusters of galaxies are pervaded by a tenuous gas that has been heated to temperatures up to 100 million degrees. The mass of this gas is comparable to that in the visible galaxies in the cluster, so X-ray observations have proven to be a sensitive method of detecting and studying such problems as the effect of the cluster environment on the member galaxies, the evolution of clusters as a whole, and the mass distribution in clusters.

Galaxies come in all sizes and shapes: majestic spirals, ruddy disks, elliptically shaped dwarfs and giants, and a menagerie of other more bizarre forms. Most currently popular theories suggest that conditions prior to birth-such as the mass of the protogalactic cloud, its size and rotation- largely determine whether a galaxy will be large or small, spiral and elliptical. However, when galaxies are crowded together, environment becomes more important than heredity.

The galaxies in the crowded central regions of rich clusters are constantly being pulled this way and that by the gravitational force fields of nearby galaxies. In addition, they are blasted and scoured by a hot wind created by their motion through the thin but pervasive hot gas in the cluster. These harsh conditions play havoc with a galaxy, so much havoc, in fact, that if our galaxy had happened to be in the central regions of a cluster rather than where it is, the Sun probably would never have formed.



Cluster of galaxies in the constellation Virgo is typical of a large but irregular cluster containing more than 1000 individual galaxies.

Cluster of galaxies in the constellation Virgo is typical of a large but irregular cluster containing more than 1000 individual galaxies. In this negative print of a photograph made with the 48-inch telescope on Mt. Palomar, the galaxies can be recognized by their fuzzy images. The large galaxy within the circle is the giant elliptical galaxy M87.


To understand this, let us take a closer look at the Virgo cluster. It is about 60 million light years away. Irregular in shape and covering about 100 square degrees in the sky (roughly the size of this book held at arm's length), the Virgo cluster contains at least 1000 spiral, elliptical, and disk-shaped galaxies. Its brightest members are giant elliptical galaxies, commonly identified by the numbers M84, M86, and M87, from the catalog of the French comet hunter Charles Messier (1730-1817). These colossal galaxies contain trillions of old stars, but few young stars, or clouds of gas such as the Orion Nebula in our galaxy.

On the other hand, the spiral galaxies on the outer edges of the Virgo cluster have bright, blue arms, indicating the presence of hot, young stars. Observations with radio telescopes indicate the presence of clouds of gas, too. In sharp contrast are the spiral galaxies nearer to the center of the cluster. They are redder in color, so they must have fewer new stars. They have less sharply defined arms and less gas. This trend continues into the core of the cluster, where spirals are rare. There we find mostly elliptical and disk-shaped galaxies. The latter resemble spirals that have had their arms ripped off. In fact, this is probably what has happened.

[153] All galaxies are believed to have formed about 10 billion years ago. Why some of them became elliptical and some of them spiral is not well understood. It seems to have been related to the efficiency with which stars were born from the gas forming the galaxy and possibly to the rate at which the protogalactic cloud was rotating.

In elliptical galaxies the efficiency of the initial episode of star formation apparently was high, whereas in spirals the efficiency was lower and an appreciable amount of gas was left over to form a flat disk rotating around a central bulge of stars. Instabilities in this newly formed disk then generated a spiral wave pattern of hot, luminous, but short-lived stars. Over the course of 10 billion years, much of this gas has been used up by the formation of stars, first in the central regions and then over an increasingly large fraction of the disk. Eventually all the gas will be gone, and the bright spiral arms traced out by newly formed stars will gradually disappear. The central bulge will be the most prominent feature, and the galaxy will appear as a disk.

If star formation were the only means of using up the gas supply of a galaxy, a spiral galaxy should retain its pinwheel pattern for many billions of years. This is apparently the case for most galaxies. Approximately 50 percent of all the large galaxies in the universe are spirals. There simply has not been enough time for all the gas to condense into stars. Also, stellar winds and supernova explosions are recycling gas back into the interstellar medium to a certain extent.

However, in rich clusters such as Virgo, spiral galaxies are much less numerous; they amount to 30 percent or less of the total. Either they do not form as frequently in clusters or they age and disintegrate much more rapidly in the hurry burly of a rich cluster. Which is it?

X-ray observations using the HEAOs have helped to answer this question. They have shown that Virgo and other rich clusters are pervaded by a very hot low density gas. The density of this gas is highest in the center of the cluster and decreases smoothly toward the outer regions. In addition to this extended cloud of hot gas, pools of hot gas exist around the giant elliptical galaxies. These galaxies show up as bright spots on an X-ray map of the cluster.

The most massive galaxy in the cluster, M87, is located near the center of the cluster and is moving very slowly, if at all, through the intracluster gas. The X-ray source around it is caused by gas trapped in the gravitational field of this massive galaxy.

The galaxy M86, on the other hand, is moving rapidly through the cluster gas at a speed of several million miles an hour. The resulting aerodynamic drag heats and compresses the gas trapped around this galaxy, creating a brightening in X-rays. The aerodynamic drag, or ram pressure, is also stripping the gas from the galaxy as it moves through the core of the cluster. This shows up on the X-ray images as a plume of emission trailing behind the galaxy. An analogous process is the stripping of leaves from a [154] tree in a windstorm. By the time M86 has crossed the central region of the core, it will have been almost completely purged of gas.

Consider now the fate of a spiral galaxy moving from the low density outer reaches of the cluster into the inner high density region. For 5 billion years or so it has been in a region where the aerodynamic pressure is low, and the process of star formation has proceeded unhindered. The spiral arms are clearly defined, and the galaxy has an ample supply of gas from which to form more new stars.

But once the spiral moves into the center of the cluster, the aerodynamic pressure, or wind, picks up and begins to blow the gas out of the galaxy. This occurs fairly quickly, after only a few hundred million years, during which time the galaxy has traveled only about a third of the way across the cluster core. This picture is confirmed by observations that show that virtually every rich cluster contains hot gas and that three-quarters of all spiral galaxies in the cores of clusters are excessively red due to the lack of new stars. Over a slightly longer period, the remnant spiral pattern dissipates; by the time the galaxy leaves the cluster core, it will have been transformed into a smooth disk galaxy.

If our galaxy had happened to form well within the core of a cluster such as Virgo, the Sun would probably never have formed. That is because....


X-ray image of the central region of the Virgo cluster.

 X-ray image of the central region of the Virgo cluster. This Einstein Observatory image shows the area around M87. The X-rays come from a halo of hot intergalactic gas that is trapped in the gravitational field of the galaxy. (Smithsonian Institution Photo No. 80-16250)


[155] ...the Sun is a second or third generation star located in the disk of the galaxy. It was formed from the leftover gas, 5 billion years or so after the initial period of star formation. In a rich cluster, the galaxy may well have been stripped of gas, and the Sun, or stars like it, might never have been formed.

As a galaxy moves through the core of a rich cluster, it not only encounters hot gas that scours it, it encounters other galaxies that can tear it apart. Collisions and close encounters with nearby galaxies can warp, stretch, and pull a galaxy apart, producing a weird zoo of strange forms. If the collision is a one-on-one affair at moderate to high speeds between galaxies of approximately the same size, they will emerge from the encounter distorted and a little ragged, but basically intact. However, if a galaxy coasts by a much larger one in a slow, grazing collision, it can be completely disrupted and assimilated by the larger galaxy. Under the right conditions these cosmic cannibals may consume 50 to 100 galaxies, bloating up in the process to become supergiant elliptical galaxies. These galaxies show up as bright optical and X-ray galaxies at the center of rich clusters.

A cluster is a gigantic swarm of galaxies, in dynamic balance between gravitational energy and kinetic energy (the energy of motion of the galaxies). The collisions between galaxies in a cluster cause a gradual, inexorable tilting of this equilibrium in favor of gravity and a change of character of the cluster. Over the course of eons, the cluster, which may have started out with a loose irregular shape, will gradually become more compact and spherically shaped. X-ray images of clusters from HEAO 2 provide striking evidence for the different stages of this process. In effect, they give us snapshots of clusters at different stages of evolution. There are loose, irregular clusters that are dynamically young, and there are clusters that are best described as double clusters. These show two large clumps of gas and galaxies that have separately relaxed to smooth, quasi-spherical shape, but have yet to merge with each other. They are thought to represent an intermediate phase and to confirm theoretical ideas as to how clusters should evolve. Finally, there are clusters that show a smooth spherical shape, with the density of galaxies increasing toward the center.

In essence, galaxies in a cluster orbit around their center of mass in much the same way as planets and other solar system objects orbit around the Sun. In a rich cluster, the galaxies are so close together that some collisions inevitably occur. In most encounters, the galaxies do not meet head-on but are deflected only slightly as they brush past each other.

The cumulative effect of these collisions is to produce a dynamic friction on a large galaxy, slowing it down. As a result, it gradually spirals in toward the center of the cluster. Eventually, the gravitational forces that bind the stars to the infalling galaxy are overwhelmed by the combined gravity of the galaxies in the core of the cluster. Just as the ocean is pulled away from the shore at ebb tide by the Moon, the stars are pulled away from their infalling parent galaxy. If there is a large galaxy at the center of the cluster, it may [156] ultimately capture these stars. With the passage of time, many galaxies will be torn asunder in the depths of this gravitational maelstrom and be swallowed up in the ever-expanding envelope of the central cannibal galaxy.

It takes a few hundred million years for a large galaxy to drift to the center of the cluster and to be cannibalized. This may seem like a long time, but in a few billion years, a central galaxy could have swallowed 100 or more galaxies! The association of X-ray emission with these supergiant galaxies is a natural consequence of the conditions in rich clusters. Because of the high concentration of galaxies, large amounts of gas are stripped from the galaxies. As this gas falls into the gravitational well around the supergiant galaxy, it becomes compressed and heated, producing a bright X-ray source. Around some supergiant galaxies, HEAO 2 X-ray observations indicate that this gas is cooling and falling onto the central galaxy. The eventual fate of this gas is unknown. Possibly it could go into forming new stars, or perhaps it could fuel explosive activity in the nucleus (see Chapters 19 and 20).


The contours of X-ray intensity measured from two galaxies in the Virgo cluster, M84 (right) and M86 (center), have been superimposed on an optical view of the same region.

 The contours of X-ray intensity measured from two galaxies in the Virgo cluster, M84 (right) and M86 (center), have been superimposed on an optical view of the same region. The swept back appearance of the contours for M86 indicate that hot gas is being swept out of this galaxy. ©National Geographic Society-Palomar Observatory Sky Survey)



Cluster A85 with X-ray contours. X-ray intensity contours are superimposed on an optical photograph of the rich cluster of galaxies A85.

Cluster A85 with X-ray contours. X-ray intensity contours are superimposed on an optical photograph of the rich cluster of galaxies A85. The X-ray emission, which comes from hot gas trapped in the gravitational field of the cluster, is seen to be centered on bright supergiant galaxy. (Optical photograph © National Geographic Society-Palomar Observatory Sky Survey; Einstein Observatory overlays from Christine Jones; Smithsonian Institution Photo. No. 80-20241)


Galactic cannibalism also explains, in a natural way, another heretofore puzzling fact about rich clusters, namely, that there are very few, if any, bright galaxies in these clusters other than the central supergiant galaxy. That is, because the bright galaxies, which are the most massive, experience the greatest dynamical friction, they are the first to go down the gravitational well and be swallowed up by the central galaxy. Over the course of several billion years, 50 or so galaxies may have been swallowed up, leaving only the central supergiant and say, the fifty-first, fifty-second, etc., brightest galaxies. Of course, the process need not stop with 50 or 100 or 200 bright galaxies. Given time, all the massive galaxies in the cluster will be absorbed, leaving a rather sparse cluster composed of a supergiant galaxy surrounded by clouds of small dim galaxies, like a swarm of flies around the head of a dozing lion.

In a small, compact system of 50 to 100 galaxies, the evolution toward a cannibalistic supergiant galaxy will proceed 100 times faster than for clusters of 10000 galaxies. We should be able, then, to find a system in [158] which this transformation has already occurred. The small group of galaxies shown in the photograph on the preceding page is apparently just such a system. A central supergiant galaxy is surrounded by a few small galaxies. HEAO I X-ray observations imply that the central galaxy is enveloped by a cloud of hot gas containing as much mass as 10 large galaxies. The visible galaxies could not have supplied this mass. It must have come from galaxies that have long since been swallowed by the supergiant.

Returning closer to home, what about our Local Group of galaxies? Will our Milky Way Galaxy devour the small galaxies around us? Probably. Calculations indicate that the Large Magellanic Cloud should fall into our galaxy in a few billion years. When that happens, there will be few collisions among the stars of the two galaxies, but collisions between vast clouds of gas will be commonplace. Shock waves will rumble through the clouds, setting the heavens ablaze with candescent gas and triggering bursts of star formation. Some of these stars will rush through their evolution in a million years or less before exploding as supernovas. It will be a dazzling and perhaps a very dangerous time to be living in this galaxy.