Abstract
ABSTRACTA total of 10 000 simulations of 1000 particle realizations of the same cluster are computed by direct force summation. After three crossing times self-gravity has amplified the original Poisson noise more than tenfold on large scales, and the amplification is still far from complete. The cluster’s fundamental dipole mode is strongly excited by Poisson noise, and this mode makes a major contribution to driving diffusion of stars in energy. The diffusive flow through action space is computed for the simulations and compared with the predictions of both Chandrasekhar’s local-scattering theory and the Balescu–Lenard (BL) equation. The predictions of local-scattering theory are qualitatively wrong because the latter neglects self-gravity. These results imply that local-scattering theory can account for only a fraction of a cluster’s relaxation. Future work on cluster evolution should employ either N-body simulation or the BL equation. However, significant code development will be required to make use of the BL equation practicable and the way forward may be to merge BL theory with local scattering theory so fluctuations of every scale are efficiently handled.
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