OPTICAL TRACERS OF SPIRAL WAVE RESONANCES IN GALAXIES .2. HIDDEN 3-ARM SPIRALS IN A SAMPLE OF 18 GALAXIES

Computer-enhanced galaxy images reveal underlying spiral structures with three symmetric arms that extend between approximately 25% and approximately 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. Evidence for the corotation radius is clear in the gas-rich galaxies, in the form of sharp endpoints to star formation ridges and dust lanes in the two-arm spirals. The inner 4:1 resonance is also evident in some cases as a pair of spurs exactly midway between the main spiral arms and diametrically opposite to each other in the galaxy. All of these resonance fits are in near-perfect agreement with the ratios of resonance radii determined theoretically from rotation curves for each galaxy, and so they define the spiral pattern speed for each galaxy with some redundancy. The pattern speeds for the m = 2 and m = 3 spirals are the same to within 10%. We interpret these results to imply that most of the two-arm spirals are self-sustaining wave modes with inner wave reflections from bars inside corotation or evanescent regions where the nonbarred spirals wrap into a circle. The asymmetry in the main spirals produces an m = 1 component which beats with the m = 2 component to drive the observed m = 3 spiral exactly between its absorbing resonance limits. In this interpretation, the three-arm spirals are not independent wave modes with their own amplifiers and reflective barriers, but are instead driven waves and a likely source of energy loss from the two-arm mode. The observation of relatively weak three-arm spirals in the interacting galaxies NGC 5194 (M51) and NGC 3031 (M81) implies that three-arm spirals in general may take several revolutions to build up once an asymmetric two-arm spiral appears. In that case, tidal interactions may first stimulate a two-arm mode, forming a grand design spiral when the companion is still close, but in time the morphology should change to a multiple-arm or irregular-looking galaxy because of the increasing contribution from the three-arm component.