Galaxy Merger and Damped Oscillation?

[FallenApple]

So it is a known fact that the Andromeda galaxy will collide and merge with the Milky Way galaxy.


In the video it shows the collision. Time 1:40

My question is how does it take only 3 collisions for the merger to take place?

First all, can we assume that if no stars/dark matter/planets from one of the galaxies affects the stars/dark matter/planets from the other galaxy , the galaxies would just keep passing through each other in an infinite oscillation? My reasoning is that if there is no internal friction, then energy is conserved, if we regard the two galaxies and objects moving with a velocities associated with their respective CMs and internal disturbance as something analogous to transfer of heat for collision between classical rigid objects. Seems to make sense classically, but I think GR needs to be considered as well, and I don't know how this would affect things.

Second, the oscillation seems to be heavily damped. We know the "friction" doesn't comes from actual collisions of planets and stars, as explained by the video since the distances between celestial objects are quite large.

So does this mean that the cause of the "friction" or internal disturbance is due to them just falling into orbit with each other,?

 

[oz93666]

Collide is not the right word , they are passing through one another ...But I agree , it would seem , at first consideration , that they should pass through one another and oscillate back and forth many more times ...If they don't , it could be to do with the gaseous atmosphere which galaxies have , only a density of about one atom per cubic meter , but as stars from the other galaxy pass through this gas (and dust) it will be absorbed by the star , and change it's momentum.

 

[Bandersnatch]

Here are some materials I've found:
http://www.astro.caltech.edu/~george/ay21/eaa/eaa-mergers.pdf

What causes the rapid merging of the two galaxies? Certainly there is a transfer of orbital energy to internal motions of the stars in each galaxy—it is this energy transfer that provides the energy necessary to launch the tidal tails. The amount of energy carried away by the tidal tails amounts to only a few per cent of the total orbital energy, however; if this was the sole energy sink, interacting galaxies would slowly spiral together over many orbital periods, rather than exhibit the rapid merging seen in figure 2. Indeed, early investigations into the collisions of galaxies argued that subsequent mergers would be quite rare, unless the collision happened to be
extremely penetrating. In fact the luminous portions of galaxies represent only a small fraction of the total galactic mass—galaxies
are embedded in massive, extended ‘dark matter halos’. These dark matter halos extend to many tens or even hundreds of kiloparsecs, such that encounters where the luminous galaxies seem to pass by one another may in fact be penetrating encounters for the dark halos. When this happens, dynamical friction ensures rapid orbital decay. As the galaxies pass through each other’s dark halos, they set up a trailing wake in the halo mass distributions, creating a gravitational drag on the galaxies’ relative
motion. As a result, energy and angular momentum are transferred from the binary orbit to the internal motions of the dark halo—the orbit decays and the halos are spun up. It is this ability for dark matter halos to absorb orbital energy and angular momentum that makes galaxy mergers possible; without halos, mergers would be rare indeed.

https://en.wikipedia.org/wiki/Dynamical_friction

Googling Dynamical Friction nets many more relevant results.