‘But there are some astronomical objects much larger then the galaxies. What about their formation? I need this information to construct the database of cosmology.’

‘We believe that the clusters of galaxies occurred after the formation of galaxies themselves. But the galaxies evolutes into different statuses while undergoing agglomeration.’

EVOLUTION OF THE UNIVERSE>THE EVOLUTION AMOUNT GALAXIES>ELLIPTICAL AND SPIRAL GALAXIES

Some galaxies, notably elliptical galaxies, are fairly round and are often concentrated in rich clusters. Others, such as spiral galaxies, are very flat and tend to be more uniformly distributed in space. What are the implications of this morphological distinction for a theory of galactic evolution?

Imagine a massive collapsing cloud that is destined to become a great cluster of galaxies. Or, taking the hierarchical-clustering viewpoint, we can simply imagine a region where the frequency of occurrence of protogalaxies is much higher than the average density of these systems. A region destined to become a great cluster must contain a larger number and a higher density of galactic fragments than a smaller cloud contains. We have already seen that they could have survived mutual collisions, once they had subdivided into stellar fragments. Collisions of this sort would have occurred more or less simultaneously with the fragments’ detachment from the large cloud.

Simple arguments about star formation suggest that typical fragments were likely to be fairly flattened systems. Densities in regions of greatest compression would continue to rise, until subfragmentation into stars could occur. Such high densities were first attained when regions of the cloud collapsed into pancakelike structures. Thus, the collapsing cloud remained gaseous until galactic pancakes developed. Once pancakes formed, rapid fragmentation into stars probably followed. Since angular momentum tends to be conserved, the motions acquired during collapse in the direction perpendicular to the plane of rotation are preferentially dissipated. This process resulted in the formation of flattened, disk like systems of stars.

As mentioned previously, initial collisions between such fragments may have led to coalescence. Protogalaxies could not have developed large relative velocities until they had fallen through the cluster center, and early collisions would tend to be rather gentle and sticky affairs. The interacting protogalaxies merge together, and the resulting merged system would rapidly become spheroidal, like an elliptical galaxy. After a few close encounters and ensuing mergers had occurred, a protogalaxy would have acquired sufficient speed to subsequently survive any collisions, passing more or less unscathed through other galaxies.

Even if mergers are not the dominant process in elliptical galaxy formation, another effect will tend to create round galaxies in dense clusters where collisions occur. There is a small acceleration of the stars in the protogalaxy by the tidal forces generated during such an encounter. This effect will tend to strip stars from the loosely bound outer regions, or halos, of galaxies. There is also a small transfer of energy from the relative motion of the two systems into the internal motions of the stars. If stars in an initially flat galaxy are induced to move more rapidly, the system must thicken. We can visualize this process as a heating of thin systems. Disks are initially cold because the random motions of stars, which are responsible for the thickness of a stellar disk, are relatively low compared with the rotation velocity of the stellar system, which accounts for the radial extent of the disk.

Away from rich clusters of galaxies, we would expect collisions of protogalaxies to occur much less frequently. These galaxies should, for the most part, remain flat if they are flat initially. We indeed observe that elliptical galaxies are usually found in clusters; outside clusters, relatively few ellipticals are observed. However, we would expect the old components of galaxies to be spheroidal. According to the hierarchical-clustering theory, if galaxies formed by mergers of many smaller substructures, it is likely that round systems rather than highly flattened systems were produced. Star formation was initiated before any significant degree of flattening could occur, and the resulting galaxies would tend to be spheroidal rather than disk shaped. It is only later that the disk develops.

In fact, a flattened subsystem or disk develops naturally. As the stars evolved, they shed considerable amounts of gas within galaxy clusters, interactions among galaxies ensured that this gas would be swept out into the intergalactic medium. Outside clusters, however, the gaseous debris would spiral in toward the galactic centers, tending to collect into a disk, where it subsequently would fragment into a second generation of stars. In this way, disk galaxies are thought to form outside the great clusters, and predominantly elliptical galaxies are thought to form within the clusters. More over, since both the disk material and the intergalactic gas are secondary ejecta of stellar origin, we would expect them both to be relatively abundant in heavy elements, which have been synthesized and recycled by successive generations of the stars. In fact, the disk populations of spiral galaxies have abundances of heavy elements that, on the average, approach that of the sun. Recent evidence suggests that the hot intergalactic gas observed in rich galaxy clusters also possesses near-solar abundances of heavy elements.

Let us return to the morphological distinction between spirals and ellipticals. Elliptical galaxies acquired their name because their light distribution (or more precisely, the contours of constant brightness) appears to have a smooth elliptical shape. No very flattened elliptical galaxies are found. The most extreme cases (known as E7 galaxies) are flattened in the ratio of 3 to 1. Spiral galaxies, however, are dominated by a highly flattened disk with a distinctly different light distribution from that of an elliptical galaxy. Spirals are dominated visually by their spiral arms, which consist of gas and young stars, but these form an insignificant part of the mass distribution of the disk. The disk is uniform in surface brightness toward the center and falls off in brightness rapidly at a rather well-defined edge. In contrast, an elliptical galaxy gradually decreases in intensity from the center until it fades away and becomes undetectable against the sky background.

Why are there no very flattened elliptical galaxies? The answer is almost certainly contained in the issue of stability. A galaxy that is too flat will not maintain a symmetrical shape for many rotation periods. Instead, it will develop a central barlike condensation. Many spiral galaxies have central bars that may be manifestations of this phenomenon. Most other spiral galaxies, including the Milky Way and the Andromeda galaxies, are probably stabilized by the presence of massive spheroids of old stars that dominate the gravity of the system in its inner regions. The Sombrero galaxy is a beautiful example of a spiral galaxy with a dominant spheroid.

Nature evidently will not tolerate the presence of highly flattened ellipticals. If such systems form, they probably become unstable. A central condensation of stars develops, which may eventually settle into a disk-shaped system. If these systems are able to retain their gas and continue to form young stars, they may develop a spiral structure and be recognizable as spiral galaxies. Consequently, spiral galaxies can develop later than (as well as during) the initial collapse phase of a protogalactic cloud. This development could most easily happen outside rich clusters, where collisions with other galaxies or with hot intracluster material would not strip spiral galaxies of gas. Inside clusters, we might expect to find disklike galaxies that have been stripped of gas and are devoid of new stars or of prominent spiral structure. Such galaxies are indeed observed, and in many of their properties they are intermediate between ellipticals and spirals. These galaxies are designated SO. Although SO galaxies are highly flattened systems, they share some characteristics, such as color and spatial distribution, with ellipticals. However, whether most SO galaxies have formed from stripped spirals or instead owe their morphological characteristics to the conditions in the protogalactic gas cloud from which they condensed is a matter of continuing debate. The issue is one of nature or nurture, and is unresolved.