Spiral structure summary and Warps
I finished reading the introduction to spiral structure. This entry is a very brief overview.
Observations: The vast majority of spiral arms are observed to be trailing the rotation of the stars (i.e. the relationship between the pattern and rotation direction is the same as in the case of hurricanes and cyclones on Earth). Most spiral galaxies containing prominent arms only have two spiral arms, but some have more. The number density of all stellar types is higher in the arms, but the distribution of young blue stars is much narrower than the older stars.
Anti-spiral theorem and consequences: If spiral galaxies are in a steady state, the cause of the spiral pattern cannot be solely time-reversible physics. Otherwise, trailing patterns and leading patterns would be equally likely. If time-reversible processes are the cause of the arms, then spiral galaxies are not in a steady state.
Lin-Shu hypothesis: Spiral arms are quasi-static density waves. The density wave is caused by a combination of orbital resonances in a rotating potential and instabilities which arise in thin (quasi 2D) potentials. The Swing Amplifier helps to enhance the pattern. Stars and clouds of gas drift into and out of the arms but the overall pattern is relatively fixed.
Swing amplification: A disturbance (density perturbation) which is arranged in a leading, rather than trailing, pattern unwinds, then propagates through the central part of the galaxy and emerges as an amplified disturbance with a trailing pattern. This was not explained very well in the text.
Some alternatives to Lin-Shu: Spiral arms could be transient and in a state of constant renewal. There is a “detonation wave” theory that states that star formation induces more star formation and thus the spiral arms are like the front line of a forest fire. Some spirals could be induced from the recent interaction between galaxies or from the perturbations due to a satellite galaxy. The spirals could also be caused by the rotating oval or bar-shaped potential at the center of the galaxy.
Warps: Disk galaxies are observed to be “warped” on their outer edges. If a cylindrical coordinate system is defined at the center of the galaxy with
perpendicular to the disk, the matter in the outer regions is seen to be displaced in the 
direction in some regions and in the 
direction in other regions. The cause of this warping is not understood.