38 - Violent relaxation

Summary

he… flung himself upon his horse and rode madly off in all directions.

Stephan Leacock

Introduction and basic physical ideas

Is it paradoxical that relaxed nearly spherical galaxies and clusters of galaxies exist? Zwicky (1960) thought so and proclaimed this a major problem of extragalactic astronomy. The difficulty comes from noticing that the two-body relaxation time τ_R, given by Equation (2.11), for these systems is orders of magnitude longer than the Hubble age of the Universe. If clusters or galaxies did not form near equilibrium, two-body relaxation would not have led them to their present state. And it would seem rather contrived for their initial state to be their final state, like Athena springing from the head of Zeus.

The solution of this difficulty comes from noticing that other relaxation mechanisms exist. Indeed, the further the initial state lies from the relaxed system, the more powerful its relaxation is. King (1962) suggested qualitatively that such a relaxation process must exist and Hénon (1964) confirmed it for a restricted spherical case using numerical simulations. Lynden-Bell's (1967) work pioneered our quantitative understanding of the subject and Shu (1978) later clarified some aspects.

The basic idea behind violent relaxation is that the initial state is so far from equilibrium that large scale collective modes govern its early evolution. Since the initial distributions of masses and velocities are very irregular, individual objects (galaxies or stars) are scattered mainly by groups of objects, rather than by other individual objects.