i can not help reading the agreements in the posts here to thinking why the vortexes would form and that is by then the collection of particle form transient are coming together in a directional manner of a mass and can this be the result of two black holes impacting out of nothing less for example?
The shape of a ball elongated by attraction before impact and causing the vortex effect as the merge takes place by directional factors and the shape of the transient form not to mention the speed which in the case of two galaxies in a lesser speed after a spiral creation where impact and the energy used in change of direction has slowed the mass to its ability to do more them bump into each other instead of smashing?
Two spiral galaxies where one going through the other does give a perception of this effect to some of Hubble's pics but maybe it is more the angle that counts when they interact and what then so moved them to inter sec from different directions can only be they were the product of an interaction vortex formation themselves and acquired directional change on a galactic level ..
excuse my pondering
Peace! Graham.
PS no I'm smarter then that ... logic dictates that the jet of transient particle form are the form in direction to another particle form not necessarily in motion but possibly more static even and either way the impact is smashing an primary.