¡¥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.¡¦
Doradus, from HST
Here we must pause and review the general characteristics of observed galaxies. If we restrict our attention to the more prominent galaxies, those that make a substantial contribution to the average luminosity density of the universe, then it is apparent that galaxies span a very limited mass range. The Milky Way galaxy has a mass invisible stars amounting to about 100 billion solar masses. Other galaxies that are easily observable appear to have masses that lie within a factor of 100 of this mass. It would seem that our galaxy is neither exceptionally large nor very small. The smallest galaxies often have low surface brightness and are difficult to detect. A very substantial number of dwarf galaxies may well be present in the universe, although in aggregate they contribute very little luminosity and probably very little mass to the mean density.
Many galaxies also appear to be fairly uniform in size. The Milky Way has a radius of about 30,000 light-years. Most other spiral galaxies have similar dimensions. Elliptical galaxies exhibit a wider range of masses, and irregular galaxies are often slightly smaller. Any satisfactory theory of galactic evolution must provide an explanation for these properties of the galaxies.
The process of gaseous fragmentation of a massive cloud does lead to the formation of galaxy-sized fragments. The breakup of the original gas cloud into smaller fragments is not in itself, however, a sufficient condition for the fragments to survive. The fragments could subdivide further, or they could collide with one another and be destroyed as the cloud collapses. Such turbulence would involve only transient structures, and we are searching for structures that can survive the collapse.
The emission of photon by the downward transition of electron
The key to survival is the ability of a fragment to radiate away the energy contained in the random motions of the atoms. This capacity enables it to contract and become a more cohesive, tightly bound structure. The amount of this energy in atomic motions determines the temperature of the gas. A gas cloud cools by losing some of the kinetic energy of its constituent atoms. Individual atoms collide with one another, and the electrons, bound in the atoms, acquire energy, or become excited. This energy is almost immediately lost by radiation, for the electron in an excited atom rapidly drops to the lowest orbit it can attain. When it does this, a quantum of radiation is emitted. The net effect of atomic collisions is to convert the kinetic energy of the atoms into radiation.
the Crab Nebula
Under ordinary conditions, the radiation is free to escape from the gas cloud. We can therefore visualize a gas cloud as cooling because of the collisions between its atoms. The greater the density of its atoms, the more collisions occurs and the more radiation is emitted. Consequently, at higher densities, a gas cloud can cool more rapidly to a lower temperature.
If a gas cloud can cool, it can also contract. If it can contract, it will be less liable to disruption by colliding with other fragments. Even more significantly, if a fragment can cool rapidly enough, it will be capable of further fragmentation, because the role of gravity relative to pressure becomes progressively more dominant. It will continue to subdivide into many smaller subfragments, which eventually form stars.
We know from observations that collisions between systems of stars often leave the bulk of the stars unaffected. Stellar systems can interpenetrate at high velocity without doing any gross damage. There is so much space between the stars in a galaxy that it is as if two ghosts were to collide. Collisions therefore play a greatly diminished role in the later stages of fragmentation, after stars have formed.
At first, the collapsing gas cloud is very tenuous and diffuse. Efficient cooling, which requires a rather high density, cannot occur. However, high densities are eventually achieved, once regions of the gas cloud collapse into thin sheets or pancakelike substructures, and efficient cooling will take place in the denser regions. A more detailed study of this process indicates that two conditions must actually be satisfied for efficient cooling to occur. The fragments must have masses less than about 1000 billion solar masses; they must also possess characteristic radii of less than about 150,000 light-years. Fragments not satisfying these constraints will be either too diffuse or too hot to cool effectively. The likely fate of such fragments is to run into other fragments and be destroyed before they can break up into stars. In other words the survival of structures of galactic dimensions is favored over more massive or larger-scale structures.