I'm still puzzled. If the density wave in our area rotates at a different speed than the Local Standard of Rest (the "average" rotational speed for stars in the solar neighborhood), this means that the density wave can pass through the solar neighbood, like a wave at sea can pass through a fleet of ships steaming in formation. If this is the case, it may be even possible for the wave to be travelling in the opposite direction that the sun and its neighbors is. It is my understanding the density wave is in the interstellar medium, the stars are not carried along with it.

We see spiral arms in galaxies because as the wave sweeps past, it compresses the medium and stars are born, and these young, hot stars are extremely bright and act as markers for the arm (there are actually just as as many stars between arms as in them, but they are fainter, older stars and don't show up as brightly. As the arms passes through these hot young stars fall back onto the main sequence or rapidly evolve and fade back into the crowd.

It appears that rotation and spirality are related, but no one knows exactly how, or at least, I haven't seen it discussed anywhere. And I see no a priori reason why the density wave might not be traveling in the opposite direction to the general rotation, i.e., a galaxy might appear to be rotating clockwise, but was actually rotating counter-clockwise. The apparent rotation can be determined by visual inspection on a photograph, the actual rotation can be detected spectroscopically by Doppler shift.

There must be published rotational Doppler data that can be compared to catalogue imagery of galaxies, it would be curious to see how the two compare.