Optical Tracers of Spiral Wave Resonances in Galaxies: II. Hidden Three-Arm Spirals in a Sample of 18 Galaxies

Computer-enhanced galaxy images reveal underlying spiral structures with three symmetric arms that extend between ∼25% and ∼85% of the outer radii of the two-arm spirals. Many of the galaxies containing multiple arms or other complex spirals are clear superpositions of simple two- and three-arm structures. The three-arm spirals appear to extend exactly from the inner to the outer 3:1 resonances in most cases, and the more prominent two-arm spirals begin outside the inner 2:1 resonance and extend to the outer 2:1 resonance