Transfer of Angular Momentum in Astrophysical Discs

Doktorsavhandling, 1997

In this thesis a study of astrophysical discs, with emphasis on galactic discs, is presented. The frictional force acting on a small satellite galaxy, moving on a co-planar circular orbit in a galactic disc, is shown to be only weakly dependent on the velocity dispersion and the self-gravity of the disc. Using these facts, a simple analytical expression for the orbital decay time of satellite galaxies is derived. The predictions of the analytical expression are compared with the orbital decay times obtained in numerical simulations, and good agreement is found. The galactic disc, rather than the halo, is shown to be the dominant cause of orbital decay for satellites on co-planar orbits. The interaction between a bar and the other components of a disc galaxy is studied by means of fully self-consistent numerical simulations using a GRAPE device. It is demonstrated that the formation of an ILR may somewhat slow the decrease of the pattern speed of the bar, but also that the detailed properties of the disc have only a moderate effect on the rate of slowdown of galactic bars. Using numerical simulations, the blazar OJ 287 is studied in the framework of the binary black hole model. In the simulations, the number of particles falling into spherical regions around the two black holes is used to represent the light curve. By fitting the times for the outbursts in the observed light curve to the times obtained in the numerical simulation, a set of orbital parameters for OJ 287 is obtained. Using these orbital parameters, the times of occurrence of future outbursts of OJ 287 are predicted. The predictions are shown to agree very well with the outburst during 1995-1996. The quality of an N-body simulation is to a great extent determined by the value of the softening used in the simulation. In the final paper, an investigation concerning the optimal value of the softening length is conducted, using several different force calculation methods and different numbers of particles. The optimal value of the softening is shown to be smaller than that which can normally be attained in grid codes