Other
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt.........9s&link_type=abstract
Thesis (PH.D.)--BOSTON UNIVERSITY, 1995.Source: Dissertation Abstracts International, Volume: 56-04, Section: B, page: 2078.
Other
Scientific paper
Galaxies, because of their large stellar populations but low number densities, are collisionless systems, i.e., significant physical processes occur on time scales short compared to the collisional relaxation time. In an N-body simulation the relaxation time scales approximately linearly with N. In a galaxy this time scale is approximately 10 ^5 times the age of the universe. Thus the use of large N is necessary for the simulations to imitate the behavior of the physical system for realistic durations. It has also made possible a detailed kinetic theoretic analysis of the simulation results. The program will also be easily adaptable to the simulation of other collisionless systems, such as many plasmas. An N-body code for the implementation of 2 dimensional simulations of galactic dynamics on massively parallel supercomputers is presented. This code is capable of performing simulations containing several million particles, as compared to earlier work which used a maximum of a half million particles. Useful results were obtained in run times of only a few hours on a small (32 node) Connection Machine 5. An empirical relation between N and the duration of spiral structure in the simulation is presented. It is found, in contrast to earlier results, that for N >= 10^6 and realistic initial conditions, spiral structure persists for several tens of galactic rotations, a time comparable to the age of the universe. For the first time the apparent relaxation of a stellar disk is related to the behavior of kinetic quantities, the entropy function, int f ln f, and the velocity distribution functions. The governing kinetic equation, the Vlasov equation, has no collision term to drive the system to equilibrium. Nevertheless, for suffiently cool initial conditions the disk is found empirically to approach local thermodynamic equilibrium. The entropy increases monotonically and the late time distribution functions of the stellar velocity components are, to a good approximation, local, anisotropic Maxwell-Boltzmann distributions. These results are obtained even when the initial velocity distribution is non-Maxwellian and are in good agreement with the 1968 prediction of Shu.
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