FAST: A Fully Asynchronous Split Time-Integrator for a Self-Gravitating Fluid

Abstract

Abstract We describe a new algorithm for the integration of self-gravitating fluid systems using SPH method. We split the Hamiltonian of a self-gravitating fluid system into the gravitational potential and others (kinetic and internal energies), and used different time-steps for their integrations. The time integration was done in the way similar to that used in the mixed-variable or multiple stepsize symplectic schemes. We performed three test calculations. The first test was spherical collapse, and the second test was an explosion. As the third test, we also performed a realistic test, in which the initial model was taken from a simulation of merging galaxies. In all test calculations, we found that the number of time-steps for gravitational interaction were reduced by nearly an order of magnitude when we adopted our integration method. In the case of the realistic test, in which the dark-matter potential dominates the total system, the total calculation time was significantly reduced. The simulation results were almost the same as those of simulations with the ordinary individual time-step method. Our new method has achieved good performance without sacrificing the accuracy of the time integration.