Events at Physics
The theory of spiral density waves had its origins about six decades ago in an attempt to reconcile the winding dilemma of material spiral arms in flattened disk galaxies. We begin with the earliest calculations of linear and nonlinear spiral density waves in disk galaxies, in which the hypothesis of quasi-stationary spiral structure (QSSS) plays a central role. We then review the observational implications and tests, in which the prediction of the nonlinear compression of the interstellar medium and its embedded magnetic field was the earliest success, and the failure to detect color gradients associated with the migration of OB stars whose formation is triggered downstream from the spiral shock front seemed to be the earliest failure. We give the reasons for this apparent failure with an update on the current status of the problem of OB star formation, including its relationship to the feathering substructure of galactic spiral arms and giant associations of atomic and molecular gas. Infrared images can show two-armed grand-design spirals, even when the optical and UV images show flocculent structures. We suggest how the nonlinear response of the interstellar gas, coupled with overlapping sub-harmonic resonances, might introduce chaotic behavior in the dynamics of the interstellar medium and Population I objects, even though the underlying forces to which they are subject are regular. We then move to a discussion of resonantly forced spiral density waves in planetary ring and its relation to the ideas of disk truncation, and the shepherding of narrow rings by satellites orbiting nearby. The back reaction of the rings on the satellites led to the prediction of planet migration in protoplanetary disks, which has had widespread application in the exploding data sets concerning hot Jupiters and extrasolar planetary systems. As our final topic, we return to the issue of global normal modes in the stellar disk of spiral galaxies and its relationship to the QSSS hypothesis, where the central theoretical concepts involve waves with negative and positive surface densities of energy and angular momentum in the regions interior and exterior, respectively, to the corotation circle; the consequent transmission and over-reflection of propagating spiral density waves incident on the corotation circle; and the role of feedback from the central regions. We review self-consistent theoretical calculations of slowly growing normal modes in collisionless stellar disks. N-body simulations show that the growth of such modes can saturate at finite amplitudes without the collisional damping of a gaseous interstellar medium as long as N exceeds 3 x107. Lastly, we discuss how the amplitude modulation predicted for the destructive interference of oppositely propagating waves that form standing wave patterns may have been observed in deep infrared images of nearby spiral galaxies.