Bar-induced mass relocation in galactic discs

Artikel i vetenskaplig tidskrift, 1999

The appearance of a bar in a galaxy considerably changes the mass distribution within the galactic disc. We have therefore performed a detailed study of the mass relocation within a stellar Kuzmin disc using two-dimensional numerical simulations with fixed 3D-potentials representing the bulge and the halo. We have started from a fully axisymmetric stellar distribution and followed the galaxy through bar formation and subsequent evolution. Once the bar has formed, the radial surface density profile will split up into three domains. The two innermost domains both have exponential profiles but with very different slopes. The outermost domain remain Kuzmin-like. Except for the very centre, the inner parts of the galaxy are depopulated out to a distance of approximately twice the length of the bar. The region just outside the bar experiences the most severe depopulation; in the presented simulation, the stellar density in this area falls to one-third its original value. All stars orbiting just outside the bar will be strongly perturbed on each orbit. Whether the star will be accelerated into a more eccentric orbit or decelerated into a less eccentric one depends on the position of the pericentre passage relative to the bar position. Circular orbits within this region are unstable. Stars from the region are thus spread out all over the disc. The stellar density in the outer parts of the galaxy thus increases. Depending on their original orbits, the stars will belong to either of two dynamical populations; stars originating in the outer parts will orbit on roughly circular orbits, while stars ejected by the bar from the inner parts of the disc will move on highly eccentric orbits. Stars can even be ejected into such extreme orbits that they escape from the galaxy. Barred galaxies can thus make a minor contribution to the population of intergalactic stars found in galaxy clusters without requiring near-collisions of galaxies.