Gravitational radiation from colliding clusters: Newtonian simulations in three dimensions

We study gravitational radiation from colliding star clusters using a Newtonian N-body code. We can handle fully three-dimensional cases, which cannot yet be treated reliably with existing hydrodynamic codes for fluid stars. Comparing colliding axisymmetric star clusters to colliding axisymmetric fluid stars provides information on the importance of shock waves in the generation of gravitational waves. Comparison of axisymmetric and nonaxisymmetric star cluster scenarios then provides information on the importance of asymmetry. We also compare our numerical results with simple analytic models. We find that two important effects reduce the radiation from extended objects relative to one's expectation from simple point-mass models: phase incoherence (destructive interference) and collisionless dissipation (violent relaxation). Because of phase incoherence, the gravitational radiation from the head-on collision of two fluid stars can actually be less than the corresponding emission from colliding clusters, despite the absence of shocks in the collisionless case. Our calculations include head-on, free-fall collisions, nonaxisymmetric parabolic encounters, hyperbolic collisions, and the tidal disruption and merger of close binary clusters.