Editor's Note: Jessica Shugart is a science communication graduate student at the University of California, Santa Cruz.
Billions of years after going on a cannibalistic binge, our own Milky Way galaxy has been implicated by the stale crumbs it left behind.
Astronomers at the University of California, Santa Cruz, used Hubble Space Telescope data to spot the crumbs - ancient stars thought to be remnants of a dwarf galaxy engulfed by our hungry Milky Way when it was still young.
The finding, to be published in the upcoming issue of "The Astrophysical Journal," supports the hypothesis that the Milky Way grew by pulling in smaller galaxies and claiming them as its own.
The researchers found the stars while looking at data from the Andromeda galaxy - the next big thing the Milky Way is destined to overtake. In about 4.5 billion years, the two are set to meet up and form an elliptical galaxy (Milkomeda?). In order for astronomers to focus on stars in Andromeda, they had to cancel out the annoying stars that orbited the outer reaches of our own galaxy.
"So when that [Andromeda] study came out, I basically asked them for their contamination," said Alis Deason, the postdoctoral researcher who led the new study.
The outcast stars exist among millions of others that form a loosely packed halo around the Milky Way. Whereas our solar system is roughly 25,000 light years from the galactic center, these castaways lurk at 80,000 light years from the core.
Deason and her team sifted through seven years of archival Hubble data to find the stars that others had thrown away. They hoped to identify remnants of a "galactic shell" - a structure seen around other galaxies that hints at a cannibalistic past.
"Basically, the halo is just a hodgepodge of stars that have come from other galaxies, usually dwarf galaxies," Deason said. "The reason these are interesting to study is they retain the memories of their initial conditions."
Ultimately, the team settled on 13 stars cruising along in the halo. That they existed there, along with thousands of other stars, wasn't the tip-off to their significance. Instead, their trajectories were what gave away their foreign past. Unlike other stars in the halo that move radially around the outer realm of the galaxy, these 13 stars appeared to hover.