THE ROLE OF RESONANCES IN THE MODAL-THEORY OF SPIRAL STRUCTURE IN GALAXIES

Grand-design spiral galaxies are argued to be dominated by one or two global modes in relatively cool stellar disks. The dynamical support of these structures depends on the mutual interaction between gas and stars in the disk, the modeling of which is quite complex. On the observational side, a very promising tool to identify the underlying structure in the older stellar disk is the new imaging instrumentation in the K band (around 2.1 mum); these infrared images are now showing a very smooth, regular, and low m (often bisymmetric) large-scale spiral structure (that is naturally interpreted as direct evidence for modes) even when the visual appearance (dominated by the gas) exhibits multiple-armed and branched structures. In the modal theory of spiral structure, resonances play a crucial, but in many respects indirect, role (e.g., the inner Lindblad resonance is expected to limit the number of self-excited modes to the ones responsible for the observed large-scale structure). I will focus here on two specific aspects that have direct observational counterparts: (1) typical location of the corotation circle for normal and for barred modes; and (2) the interpretation of observed gaps in terms of interference patterns of the waves supporting the modes.