The dwarf galaxy that could: SagDEG.
Multiple tidal interactions ( 5.7 Million Years, 3 MY, 1.9 MY, ago ) between the larger mass MIlky Way galaxy and a smaller dwarf
Elliptical ( 1% mass of the MW ) may not only have formed some of its SPIRAL ARMS, but also the BAR structure of the Milky Way—deduced, and verified, initially by the UMass Astronomy Dept. That is, the MW is a barred spiral.
Stripped of most of its mass, SagDEG (Sagittarius Dwarf Elliptical Galaxy) has a dozen
Or so globular clusters, some star clusters, and stars remaining, all to be eventually cannibalized by the MW as it continue to gravitationally spiral in a polar trajectory Into our assimilating galaxy.
Until then, it will continue to plunge in and out of the galactic plane on a galactic time scale, in smaller and smaller oscillations, as its mass is stripped further with each oscillation. Until SagDEG is totally assimilated.
PS some has suggested that because of the misalignment of the solar plane with the galactic plane, that the sun
was one of the stars stripped from the dwarf galaxy during one of its passages through the Milky Way early on. That is to say, we are aliens from another galaxy. LOL