Evolution of spiral structure in an interacting triple galactic system: I. M 31-type systems

We investigate the effect of two small companion galaxies in the creation and evolution of spiral arms in a disk galaxy. Both companions move in circular orbits around the main galaxy. Using a two dimensional self-consistent computer simulation code we find that, when there is only one companion, one leading or two trailing spiral arms are formed, depending on the rotation of the companion (retrograde or direct respectively). In the case of two companion galaxies, when the companions move in opposite directions, three spiral arms are formed, one leading and two trailing. The appearance of the above structure suggests that each companion acts independently on the main disk galaxy. The created spiral arms may intersect each other, resulting in the disappearance of the weaker arm. © European Southern Observatory