When galaxies collide

A dwarf galaxy called Sagittarius can be credited with giving the Milky Wayits signature spiral arms.

Sagittarius struck our galaxy some 1.9 billion years ago. It then looped over the galactic "north pole" and struck again about 900 million years ago. It is heading back right now, on course for a third clash in 10 million years or so. New Scientist

There is a neat video simulation. The dwarf galaxy seems to be merging with us.