Galaxy merger
The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved, but the exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research.
The result of all this violence is that galaxies tend to have little gas available to form new stars after they merge.
Galaxy pairs initially of any morphological type can be followed, taking into account all gravitational forces, and also the hydrodynamics and dissipation of the interstellar gas, the star formation out of the gas, and the energy and mass released back in the interstellar medium by supernovae.
Mergers can be categorized by the degree to which the gas (if any) carried within and around the merging galaxies interacts: In the standard cosmological model, any single galaxy is expected to have formed from a few or many successive mergers of dark matter haloes, in which gas cools and forms stars at the centres of the haloes, becoming the optically visible objects historically identified as galaxies during the twentieth century.
Modelling the mathematical graph of the mergers of these dark matter haloes, and in turn, the corresponding star formation, was initially treated either by analysing purely gravitational N-body simulations[17][18] or by using numerical realisations of statistical ("semi-analytical") formulae.
These merger history trees were combined with formulae for star formation rates and evolutionary population synthesis, yielding synthetic luminosity functions of galaxies (statistics of how many galaxies are intrinsically bright or faint) at different cosmological epochs.
[20] Independently, Lacey and Cole showed at the same 1992 conference[21] how they used the Press–Schechter formalism combined with dynamical friction to statistically generate Monte Carlo realisations of dark matter halo merger history trees and the corresponding formation of the stellar cores (galaxies) of the haloes.
[19] Kauffmann, White and Guiderdoni extended this approach in 1993 to include semi-analytical formulae for gas cooling, star formation, gas reheating from supernovae, and for the hypothesised conversion of disc galaxies into elliptical galaxies.
[22] Both the Kauffmann group and Okamoto and Nagashima later took up the N-body simulation derived merger history tree approach.
