Formation of leading spiral arms in retrograde galaxy encounters

Artikel i vetenskaplig tidskrift, 1989

The formation of spiral structures in retrograde galaxy encounters was studied theoretically and with N-body simulations. A one-armed leading spiral dominates in a disk if the tidal perturbation from the retrograde companion is large enough, and the disk is surrounded by a massive halo. The leading arm is made up of particles in slightly elongated orbits, the turning points of which outline the arm. The orbits precess in such a way that the arm structure survives while it rotates in the opposite sense to the disk rotation. From the literature it is found that very few spirals in a sample of galaxies with a large companion have leading spiral arms. A possible reason for this is that very few spiral galaxies have a halo with larger mass than the disk mass.