Book contents
- Frontmatter
- Contents
- Preface
- Introduction
- Part I Idealized homogeneous systems – basic ideas and gentle relaxation
- Part II Infinite inhomogeneous systems – galaxy clustering
- Part III Finite spherical systems – clusters of galaxies, galactic nuclei, globular clusters
- 37 Breakaway
- 38 Violent relaxation
- 39 Symmetry and Jeans' theorem
- 40 Quasi-equilibrium models
- 41 Applying the virial theorem
- 42 Observed dynamical properties of clusters
- 43 Gravithermal instabilities
- 44 Self-similar transport
- 45 Evaporation and escape
- 46 Mass segregation and equipartition
- 47 Orbit segregation
- 48 Binary formation and cluster evolution
- 49 Slingshot
- 50 Role of a central singularity
- 51 Role of a distributed background
- 52 Physical stellar collisions
- 53 More star–gas interactions
- 54 Problems and extensions
- 55 Bibliography
- Part IV Finite flattened systems – galaxies
- Index
38 - Violent relaxation
Published online by Cambridge University Press: 05 July 2011
- Frontmatter
- Contents
- Preface
- Introduction
- Part I Idealized homogeneous systems – basic ideas and gentle relaxation
- Part II Infinite inhomogeneous systems – galaxy clustering
- Part III Finite spherical systems – clusters of galaxies, galactic nuclei, globular clusters
- 37 Breakaway
- 38 Violent relaxation
- 39 Symmetry and Jeans' theorem
- 40 Quasi-equilibrium models
- 41 Applying the virial theorem
- 42 Observed dynamical properties of clusters
- 43 Gravithermal instabilities
- 44 Self-similar transport
- 45 Evaporation and escape
- 46 Mass segregation and equipartition
- 47 Orbit segregation
- 48 Binary formation and cluster evolution
- 49 Slingshot
- 50 Role of a central singularity
- 51 Role of a distributed background
- 52 Physical stellar collisions
- 53 More star–gas interactions
- 54 Problems and extensions
- 55 Bibliography
- Part IV Finite flattened systems – galaxies
- Index
Summary
he… flung himself upon his horse and rode madly off in all directions.
Stephan LeacockIntroduction 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 τR, 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.
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- Information
- Gravitational Physics of Stellar and Galactic Systems , pp. 270 - 286Publisher: Cambridge University PressPrint publication year: 1985