Theory 15/Galactic Astronomy
Spiral Pitch Is a Mass Ratio
The claim
The pitch angle of galactic spiral arms is set by the ratio of disc mass to halo mass rather than by the rotation curve alone.
01The mechanism
Swing amplification depends on how much of the disc's self-gravity can drive the wave against the halo's stabilising mass; the ratio sets the winding.
02Predictions
- P1Pitch should track the mass ratio.
- P2Rotation curve alone should mispredict.
03Refutation
Pitch tracking rotation curve would remove the halo term.