Merging two galaxies to create a new huge galaxy and making new stars in the process. Another great collaborative effort. Maybe this will be what happens to the Milky Way and Andromeda many millions of years from now.
The story from NASA (source):
Several telescopes have teamed up to discover a rare and massive merging of two galaxies that took place when the universe was just 3 billion years old (its current age is about 14 billion years). The galaxies, collectively called HXMM01, are churning out the equivalent of 2,000 suns a year.