Introduction

Recent years have seen impressive advances in the simulation of collisionless systems. However, a wide range of problems in galactic dynamics are still amenable to direct N-body integration and will therefore be discussed for completeness. This chapter also serves as a historical review of several topics that blossomed from a primitive beginning. All such problems are characterized by the employment of a small softening of the potential which reduces the effect of close encounters. We begin by describing an application of the grid perturbation method to a ring of interacting molecular clouds in the inner part of the Galaxy [Aarseth, 1988b]. This enables a realistic number of clouds to be considered. The tidal disruption of dwarf spheroidal galaxies orbiting the Milky Way has also been studied by direct means [Oh, Lin & Aarseth, 1995].

More extreme interactions of galaxies often result in the formation of one system, subsequently denoted as a remnant to distinguish this process from mergers used for hierarchical stellar configurations. Studies of galaxy interactions involving black holes have also become topical [Makino & Ebisuzaki, 1996; Makino, 1997]. This problem is particularly challenging because of the difficulty of scaling to realistic conditions. Small galaxy groups and clusters are ideal for N-body simulations if close encounters can be treated using a softened potential.