Retro/prograde flyby galaxies and their bar formation

[milkism]

TL;DR Summary

In fast flyby encounters, typically involving a smaller or equal in mass galaxy (secondary) passing by a relatively stationary galaxy (primary), a bar formation can occur in either the secondary galaxy or both galaxies. This depends on their masses and their relative motions (whether prograde or retrograde).

https://iopscience.iop.org/article/10.1088/2041-8205/790/2/L33/pdf

In section 3, it states that during a 1:1 prograde passage, bar formations are visible in both galaxies, whereas in the retrograde version, there are no bar formations in either galaxy. I would like to understand the physics behind this.

My initial thought is as follows:

During a 1:1 retrograde passage, the gravitational interactions of both galaxies are equal in magnitude, but their signs differ. Therefore, if we sum up all the gravitational forces, the net effect would be zero.

Is this partially correct (of course, aside from gravitational forces, there are other factors too) or am I completely wrong?

 

[pbuk]

a relatively stationary galaxy (primary)

Do you think there is any significance of a reference frame in which one of the galaxies is 'relatively stationary'?

I would like to understand the physics behind this.

If the physics was easy to understand we wouldn't need to spend thousands of hours of CPU time performing simulations.

During a 1:1 retrograde passage, the gravitational interactions of both galaxies are equal in magnitude, but their signs differ.

Yes, according to Newton's third law. How is this different in a prograde passage?

Is this partially correct

I don't think so. Galaxy flybys may cause perturbations in galaxy structure due to the gravitational attraction of individual stars towards the "other" galaxy. In a prograde flyby individual stars are closer to the other galaxy for much longer than in a retrograde flyby (similar to the difference between a car overtaking you and a car passing in the opposite direction) so this attraction has a much greater effect.