Numerical Studies of Extragalactic Objects. Analysis of mmVLBI Observations and Simulations of Barred Spiral Galaxies

Doktorsavhandling, 1998

Numerical studies related to high-frequency VLBI observations of quasars and dynamics of barred spiral galaxies have been performed. Data from a global 100 GHz VLBI experiment has been reduced and a radio image of the quasar 3C 446 with the highest resolution achieved to date has been produced. A new computer program package ( NEW_LSD ) is presented. This software provides a highly automatic platform for model fitting of Gaussian components to small or incomplete sets of VLBI data. Fundamental problems related to multi-parametric model fitting are analysed, as well as the strategies implemented in this software. A comparison of algorithms included in NEW_LSD shows that methods based on ``simulated annealing'' are the most successful ones. Dynamical simulations of barred galaxies have been performed with a two-dimensional, polar-grid, particle-mesh code. The results show that bars usually grow in length until they reach out to their co-rotation radii, which generally are located where the swing amplification is at maximum. There are clear trends in bar behaviour when the properties of the bulge is changed systematically. A more compact bulge gives a shorter bar rotating with a higher pattern speed. Slow-down rates are, however, rather unaffected by changes to the bulge parameters unless the bulge is very massive and compact. The surface density profile of the disc is radically altered by the bar. The stellar density increases in the very centre and in the outer parts of the disc while there is a massive depopulation just outside the bar region. This depopulation is caused by repeated perturbations from the bar, sending stars outwards on highly elliptical orbits. Stars can even be ejected to escape velocities, and barred galaxies can thus contribute to the population of intergalactic stars. An extra bar-like dynamical component, ``the oscillating feature'', may appear between the end points of the bar and its co-rotation radius. Angular momentum is continuously transferred back and forth between the bar and the oscillating feature leading to periodic oscillations in the pattern speeds of the two components. The oscillations diminish as the oscillating feature is gradually replaced by the growing bar.