We will concentrate here on two other mechanisms which have their own problems when faced with observations, but appear promising at the moment. We will describe those two mechanisms in the rest of this section.
Cylindrical rotation follows naturally.
Clearly, a whole range of such scenarios is possible, from the accretion of one or more small satellites galaxies to the merger of two galaxies of similar size. However, accretion scenarios face several problems. It seems unlikely that several small accreted satellite galaxies would all satisfy this criterion. This argues against any kind of recent accretion or merger models. Bar formation makes the orbits in the bar more eccentric and aligns their principal axes without greatly affecting the motion perpendicular to the plane.