Study on the formation and evolution of galaxies by Large-scale numerical simulations on special-purpose computers.

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1. We have developed a high-efficiency parallel implementation of the Barnes-Hut tree algorithm on parallel GRAPE system. Using this calculation code, it is now feasible to perform simulations of cosmological structure formation in the spatial scale smaller than clusters of galaxies using 〜 10^8 particles. Only one such simulation has been previously reported.2. By cosmological simulations with more than 30 million particles, we have determined the slope of the central cusp of the dark-matter halo in CDM cosmology with unprecedented accuracy and reliability. The slope is around -1.4. We have not observed any tendency of the slope to approach to ―1. This slope does not depend on the cosmological models.3. We have found the reason of the discrepancy between N-body simulations and semi-analytic calculations of the orbital evolution of satellite galaxies. The evolution of the satellite galaxies in recent high-accuracy N-body simulations turned out to be significantly slower than the result of approximate calculations using Chandrasekhar's dynamical friction formula. We found that the discrepancy is not due to the dynamical friction formula itself, but due to the common exercise of taking the size of the parent galaxy as the upper limit for the Coulomb logarithm. By taking the theoretical preferred value, namely the distance of the satellite from the center of the parent galaxy, we resolved the discrepancy. In the case of the satellite from internal degree of freedom, we found that the evolution is faster than that for analytic dynamical friction formula.