Is it known whether spiral galaxies typically (or exclusively?) rotate with the arms trailing or facing?
Intuitively it feels weird to think of the arms as facing the direction of rotation, but that's silly, of course -- it seems to have its origin in an instinctive assumption that some sort of friction with the intergalactic vacuum would make it hard for the galaxy to rotate against the barbs, which is nonsense.
I assume that for galaxies that we don't view head-on, we can measure rotation curves spectroscopically. But that's not enough; it must also be possible to determine which end of the minor axis is the far one. Dust in front of the bulge can help with that -- but in the images I have seen where it is clear which way the galaxy is tilted, it's not easy to see which way the spiral curls.
Still, I suppose there must be some galaxies where both the spiral and the rotation can be determined. What's the verdict for them?
Answer
According to this article, most spiral galaxies rotate with the spiral arms trailing. NGC 4622 appears to be an exceptional case, probably because of a past interaction with another galaxy.
Interestingly, the spiral arms seem to be transient phenomena, like density waves. A star in an arm doesn't necessarily move along with the arm; instead, the arm is a region of space that currently has a lot of young, bright, and short-lived stars. The Wikipedia article on spiral galaxies has (what I assume to be) a good summary.
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