re: The Tadpole Galaxy (APOD)
I think in collisions of this kind, the resultant "tadpole" is both galaxies. Similar to the proto-Earth, Thea collision in concept.
A galaxy is only really a "thing" like a cloud is a thing. Once two mix, there's stellar scattering as closely interacting stars fling eachother to points beyond. However, most stay associated in a galactic manner to the new structure.