Study of the evolution of dense stellar systems by large-scale simulations on special-purpose computers

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The purpose of this study is to clarify the dynamical evolution of dense stellar systems, such as globular clusters, so-called young and compact clusters, and galactic nuclei, by means of direct N-body simulations. The focus of the study is how the massive black holes grow at the center of these systems. Specifically, we placed two goals. (a) To determine the criterion for the occurrence of the merging instability, in which massive stars merge with other massive stars progressively to form supermassive stars, and (b) to identify the observable signature of massive central black holes in globular clusters and other massive star clusters. As for the condition for the merging instability, we found that the condition is that the initial dynamical friction timescale of massive stars is short (less than 3Mys) and that the initial core radius is fairly small. When fitted by a King mode, W0 must be 8 or larger. For the observational signature of the central black hole, previous observational studies looked for a cusp with slope -7/4, predicted from simple single-component model. By means of N-body calculation in which stellar evolution and stellar mass function are taken into account, we found that the central region of a globular cluster with central massive black hole appears as almost flat core.