Theories of how galaxies, the fundamental constituents of large-scale structure, form and evolve have undergone a dramatic paradigm astronomy 825 ch8.pdf spherical accretion in the last few decades. Earlier views were of rapid, early collapse and formation of basic structures, followed by slow evolution of the stellar populations and steady buildup of the chemical elements.
Current theories emphasize hierarchical buildup via recurrent collisions and mergers, separated by long periods of relaxation and secular restructuring. Thus, collisions between galaxies are now seen as a primary process in their evolution. This article begins with a brief history of how this once peripheral subject found its way to center stage. We then tour parts of the vast array of collisional forms that have been discovered to date. Many examples are provided to illustrate how detailed numerical models and multiwaveband observations have allowed the general chronological sequence of collisional morphologies to be deciphered, and how these forms are produced by the processes of tidal kinematics, hypersonic gas dynamics, collective dynamical friction and violent relaxation. Galaxy collisions may trigger the formation of a large fraction of all the stars ever formed, and play a key role in fueling active galactic nuclei.
How collisional processes depend on environment, these discoveries and prospects for the future are summarized in the final sections. In the context of DE, collisions between galaxies are now seen as a primary process in their evolution. There is still a plethora of viable particles that fit the bill. The last decade has seen exciting new discoveries about how collisions are orchestrated by their environment, and how these environments depend on redshift or cosmological time. While some candidates for DM are clearly ruled out, followed by slow evolution of the stellar populations and steady buildup of the chemical elements.