swinging both ways

do you lead, or trail?

I was reminded of this beauty for obscure social reasons...

In theory, spiral galaxies may have either "leading" or "trailing" spirals.
In practise almost all that we can measure have trailing spirals.

In trailing spirals the spiral arms flow back with the rotation, as the name indicates, whereas with leading spirals the arms open into the rotation.
For most spiral galaxies, the arms rotate differentially, the outer parts of the arm usually have lower angular velocity than the inner parts, and tend to "wind-up" into more tight multiply wrapped arms, if they are trailing. In contrast the leading arms open up more and more, if there is differential rotation, until they straighten out and wrap back into trailing arms.

So, the general tendency is for leading spirals to unwind and become trailing, so in practise we might not be surprised to find that actual spirals are mostly trailing.

With some exceptions (well, three that I have heard of), and NGC 4622 is the most spectacular.

We know NGC 4622 has leading arms, because it swings both ways.
Look carefully, the outer arms are wrapped in the opposite sense to the inner arms, one set has to be leading (er, well, unless the inner and outer disks counter-rotate, which is conceivable).

Why is this?
Not sure, there are some conjectures, but nothing rock solid.
Pretty though.

Interestingly, the original conjecture was that the inner arms were leading, but additional data in obtained the last few years suggests that the inner arms may be normal trailing arms and the outer arms are leading.
Leading arms can be excited by retrograde encounters of massive satellite galaxies, but usually this leads to evanescent and lopsided arms. Getting a symmetric grand design like this and having both senses of rotation present is very hard to arrange - most scenarios are quite contrived (symmetric, 180 degree out of phase, coplanar retrograde pairs of satellite galaxies, anyone?)