Abstract
The spiral arms of a galaxy are not just aesthetic features, but are a galaxy's means of increasing its mass and accreting gas. Indeed, matter tends to collapse onto galaxies continuously from cosmic filaments, but it must first shed its angular momentum, before it can fall towards the center and form new stars. This requires non-axisymmetric structures such as bars and spirals, which generate tangential forces and torques that can transfer angular momentum outwards.
Lin & Shu's density wave theory is based on this principle. Spiral arms are not material arms that rapidly wind and dilute through differential rotation, but density waves that travel at constant rotational speed. However, these waves are not eternal, and decay very quickly, especially in the presence of gas and shocks. We therefore need to consider mechanisms for generating and maintaining these waves. Swing amplification shows how a leading wave packet is considerably amplified when transformed into a trailing wave, when epicyclic frequency and self-gravity combine with differential rotation. We can then imagine waser cycles, analogous to masers (or lasers), where waves are amplified at the corotation, and reflected at the center, at the internal Lindblad resonance. The presence of a star bar, or the interaction between galaxies, produces the mechanisms needed to generate the formation of the first spiral wave. However, coherent density waves over the whole disk are rare, and we observe instead pieces that regenerate with the swing mechanism. Recent cosmological simulations, which contain a much larger amount of dark matter than observed in reality, tend to stabilize disks, and see fewer and fewer spiral density waves.
