Collision With Dwarf Galaxy May Cause Spiral Structure of Milky Way

According to scientists two collisions with a dwarf galaxy over the last nearly 2 billion years may have been the cause of the Milky Way’s spiral arm structure. The new findings suggested that impacts with even relatively small galaxies have played an important role in shaping galactic structure throughout the universe. In trying to explain the shape of our own galaxy, the Milky Way, with its prominent spiral arms rooted in a central bar, scientists have traditionally dismissed the influence of outside forces, even though astronomers have seen shape-changing mergers of other galaxies.  

For their study, scientists focused on the nearby Sagittarius dwarf galaxy, much of which had been ripped apart by the gravitational pull of the Milky Way, leaving debris that   forms a huge but very faint stream of stars around our galaxy. Altogether, this dwarf galaxy might have once been far more substantial, maybe 100 times more massive. However, in computer simulations scientists found this dwarf galaxy’s collision with the Milky Way might have had dramatic consequences. It may have triggered the formation of our galaxy’s spiral arms, caused the flaring seen in the outermost disk, and influenced the growth of its central bar.
source:www.space.com