Formation of rings in galactic discs by infalling small companions

We use N-body simulations to study the formation of rings in a disc galaxy bythe impact of a small spherical companion. Both barred and nonbarred targetdiscs are considered. We discuss the effect of the properties of the targetdisc (distribution of mass in the disc, velocity dispersion, etc.) as well asof the mass and orbit of the companion on the properties of the rings, such astheir amplitude, width, shape, expansion velocity and lifetime. In particularthe amplitude, width, lifetime and expansion velocity of the first ringincrease considerably with companion mass, and so does the expansion velocityof the particles in it and the total extent of the disc after the interaction.We also discuss the formation and evolution of spokes and show that they can becaused by companions of relatively small mass. In our three examples of obliqueimpacts on barred target galaxies we note important transient displacements ofthe bar, as well as changes of its pattern speed and size. An asymmetricpseudoring is formed in each case, and during the first stages of its evolutionthe bar forms part of it.