| Full text | |
| Author(s): |
Beraldo e Silva, Leandro
;
Pedra, Walter de Siqueira
;
Sodre, Laerte
;
Perico, Eder L. D.
;
Lima, Marcos
Total Authors: 5
|
| Document type: | Journal article |
| Source: | ASTROPHYSICAL JOURNAL; v. 846, n. 2 SEP 10 2017. |
| Web of Science Citations: | 1 |
| Abstract | |
The collapse of a collisionless self-gravitating system, with the fast achievement of a quasi-stationary state, is driven by violent relaxation, with a typical particle interacting with the time-changing collective potential. It is traditionally assumed that this evolution is governed by the Vlasov-Poisson equation, in which case entropy must be conserved. We run N-body simulations of isolated self-gravitating systems, using three simulation codes, NBODY-6 (direct summation without softening), NBODY-2 (direct summation with softening), and GADGET-2 (tree code with softening), for different numbers of particles and initial conditions. At each snapshot, we estimate the Shannon entropy of the distribution function with three different techniques: Kernel, Nearest Neighbor, and EnBiD. For all simulation codes and estimators, the entropy evolution converges to the same limit as N increases. During violent relaxation, the entropy has a fast increase followed by damping oscillations, indicating that violent relaxation must be described by a kinetic equation other than the Vlasov-Poisson equation, even for N as large as that of astronomical structures. This indicates that violent relaxation cannot be described by a time-reversible equation, shedding some light on the so-called ``fundamental paradox of stellar dynamics.{''} The long-term evolution is well-described by the orbit-averaged Fokker-Planck model, with Coulomb logarithm values in the expected range 10-12. By means of NBODY-2, we also study the dependence of the two-body relaxation timescale on the softening length. The approach presented in the current work can potentially provide a general method for testing any kinetic equation intended to describe the macroscopic evolution of N-body systems. (AU) | |
| FAPESP's process: | 12/00800-4 - The 3-D universe: astrophysics with large galaxy surveys |
| Grantee: | Laerte Sodré Junior |
| Support type: | Research Projects - Thematic Grants |
| FAPESP's process: | 09/54006-4 - A computer cluster for the Astronomy Department of the University of São Paulo Institute of Astronomy, Geophysics and Atmospheric Sciences and for the Cruzeiro do Sul University Astrophysics Center |
| Grantee: | Elisabete Maria de Gouveia Dal Pino |
| Support type: | Multi-user Equipment Program |
| FAPESP's process: | 14/23751-4 - Dynamics and Statistical Mechanics of dark matter halos |
| Grantee: | Leandro José Beraldo e Silva |
| Support type: | Scholarships in Brazil - Post-Doctorate |
| FAPESP's process: | 12/16835-1 - Cosmic acceleration and the dark energy survey |
| Grantee: | Marcos Vinicius Borges Teixeira Lima |
| Support type: | Regular Research Grants |
| FAPESP's process: | 16/02503-8 - Macroscopic behavior of non-relativistic interacting many fermion systems |
| Grantee: | Walter Alberto de Siqueira Pedra |
| Support type: | Scholarships abroad - Research |