VERSION|0.5.1|NAME|Steve|DATE|1365072390|CONTENT|Further to Dave Harper&#039;s comment, when any two fluid vortices collide there will be a transfer of energy from one to the other. The boundary between the two is a shear boundary. Energy is transferred through the resistance to shear (viscosity), mixing the fluids and homogenising the flow.

If the viscosity was very small, the two vortices would transfer energy very slowly between each other, and they would continue to rotate as they did before the collision for quite a long time. I guess that in the case of galaxy collisions, this &quot;interstellar viscosity&quot; is provided by gravity, and that it takes time to homogenise the &quot;flow&quot;. If that time is long compared to the relative linear velocity of the pair, they could occupy the same region of space while maintaining a large part of their original rotations.

I don&#039;t know why we see the two original galaxies in the same place. If the forces transferring rotational energy from one to the other are small, the linear movement should be unimpeded as well. It could be that their currect concentricity is a coincidence, that in a few million years these galaxies will have passed through each other and out the other side.|IP-ADDRESS|10.0.119.138, 217.36.222.79, 10.37.44.203|MODERATIONFLAG|