Inner two-arm symmetry in spiral galaxies

Most galaxies with spiral density waves, including those with multiple long arms, have two prominent symmetric arms in their inner regions, inside ∼0.5R25. Grand-design galaxies, which have two prominent arms throughout their disks, also have brighter, narrower, and more continuous arms inside this radius. Based on measurements of these morphological features in 173 spiral galaxies, and on our previous studies of optical resonance indicators, we propose that corotation is optically visible in most spiral galaxies and is located near the radius of the endpoints of the highly symmetric part of the spiral arms, approximately midway out in the disk. This places the outer Lindblad resonance at ∼R25, or ∼4 scale lengths, for most spiral galaxies. In barred galaxies, the two inner symmetric arms end at twice the bar radius, independent of the bar type. If large bars end near corotation, then the ends of the two prominent arms must be beyond corotation in these systems.