Abstract
Much is still being written about the generation of spiral density waves. Other mechanisms could be considered besides the common, swing-amplified waves. For example, the waves could be eigenmodes. In addition, many waves could develop in disks, interacting with each other in non-linear ways, such as primary and nuclear rods, or coupled rod-spiral assemblies. Waves could thus develop all the way to the center, and even emerge in the outer zones of galaxies, where HI atomic gas prevails. Harmonics could develop, such as spur-like, feather-like or branching instabilities. The magnetic field may also play a role, generating Parker instabilities that produce regular packets that form stars like pearls on a necklace. Spirals and stellar bars produce shock waves in the gas component. This can cause the gas to condense into stars, and may explain why the spiral arms are so much brighter than the inter-arms, even though the density contrast is not huge.
How to diagnose the presence of spiral density waves? If there is a strong contrast in the gravitational potential, we should see characteristic velocity perturbations as we cross the bars. These diagnoses are sometimes observed and sometimes not, perhaps due to local disturbances (supernova explosions, interactions, etc.).
Numerical simulations help us to understand phenomena such as the formation of thick disks, supernova feedback phenomena, flaring and so on.
