¡¥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.¡¦
All galaxies are rotating, but ellipticals are rotating less rapidly than spirals. The origin of galactic angular momentum can be attributed to the action of tidal torques between neighboring protogalaxies. As long as the protogalaxies are not spherically symmetric, differential accelerations are induced by neighbors, and these result in the acquisition of angular momentum. No net angular momentum is produced: one object spins clockwise, and its neighbor spins counterclockwise. The spins induced this way are very small, amounting to a rotational velocity that is perhaps only a tenth of the critical value needed for centrifugal balance against gravity.
As the protogalaxy collapses, however, it spins more and more rapidly. Exactly how rapidly depends on whether a halo of inert dark matter is present. The dark halo acts as a support against which the gas can torque as it loses energy and falls toward the inner regions. After collapsing by about a factor of 10 in radius, the gas is supported centrifugally in the plane of rotation: perpendicular to this plane, the gas distribution flattens because there is no support. In this way, a rotating gas disk develops that now fragments into stars to form a spiral galaxy.
Large Magellanic Cloud
In the absence of a dark halo, centrifugal forces do not become dominant; instead, stars form when the protogalaxy has contracted to form a flattened spheroid. The flattening of an elliptical is due not to systematic rotation but to anisotropic random motions of the stars: the motions are somewhat smaller in the direction perpendicular to the symmetry plane. A merger of two disk galaxies also produces a spheroidal stellar system, much like an elliptical galaxy. Whether the usual elliptical-galaxy formation mechanism is through loss of a dark halo, followed by contraction of the gas cloud, or through mergers is not known, but both processes are likely to occur during the formation of galaxy groups and clusters.