If the opposing spin of one set of arms was really due to an assumed collision with another (unknown) body, as suggested by the U of Alabama team, it would be most likely that the resulting motion of the contacted arm would be very unlike the opposing arm — but in reality, the arm motion of each side are quite similar to each other (nearly symmetrical). It would be most likely that the resulting motion of one arm, if contacted by an alien body, would be, at least, somewhat 'out-of-plane' with respect to the rest of the parent galaxy.

My view of NGC 4622 indicates that –
o two arms initially formed each on opposite sides of the parent body.
o they continually expanded outward around the parent at slightly greater than escape velocity.
o then when they reached about 180 deg from their origin, one continues on, but the other splits into two sub-arms.
o the outer sub-arm takes on a slight increase in escape velocity and continues a pattern away from its parent.
o the inner sub-arm slows somewhat from its original speed (decreases escape velocity) and, consequently, patterns inward toward its parent.
o this altered inner sub-arm trajectory (as well as the outer sub-arm) may now be tilted somewhat out of its original plane of motion.
o this slower inner sub-arm motion could now give the impression to an observer that it is an arm that is moving in the opposite direction from which it originated — opposite motion from the outer arms. But I believe that it is simply continuing on in its original general direction, but at a slower speed, and moving inward toward its parent. Slower than escape velocity will cause it to traject into its parent somewhat like a near vertical rocket launch from earth.

What caused the arm split? Who knows? But it could be the same object presumed by the U of Alabama team to have collided with NGC 4622 to cause the reversal in arm rotation. No one could possibly determine when it impacted, where it contacted NGC 4622, its size, shape, density, velocity, direction of motion at impact with respect to the path of NGC 4622.

It appears to me that my hypothesis (impacting body splits an arm) is certainly questionable, but it appears to be more logical than an impacting body causing a reversal in arm rotation.

Readers — what are your thoughts?

My favorite so far is the intermediate black holes from globular clusters.