Galaxies are big cannibals that constantly gobble up smaller galaxies nearby.
Scientists have just discovered new signs of this cosmic carnage in nearby galaxies, including our closest neighbor, Andromeda.
When wimpy galaxies pass near behemoths like the Milky Way, the unfortunate weaklings are slowly swallowed up, their stars integrated into the parent. But they leave behind evidence of their demise in the form of so-called tidal streams, which are trails of stars that were ripped off the small galaxies by gravitational forces.
The tidal streams trace the path the mini galaxies followed as they orbited around their parent before they were swallowed up. Scientists can use these orbits to measure how much the larger galaxies weigh and how their mass is spread out, and can even learn more about how the galaxies were formed and evolved.
"You can see these very complex systems of shells and plumes of tidal debris that mark the past accretion history of the galaxy," said astronomer Chris Mihos of Case Western Reserve University in Cleveland, Ohio.
Mihos recently found signs of tidal tails around some of the galaxies in the Virgo Cluster, a relatively nearby collection of galaxies about 50 million light-years away.
Another study, led by Puragra Guhathakurta of the University of California, Santa Cruz, also recently found sings of galactic cannibalism in the form of tidal streams around the Andromeda galaxy, only 2.5 million light-years away.
"The tidal stream gives you a window in time during which the events happened: the last couple billion years," Guhathakurta said. "If it was earlier, it's unlikely we'd still see the stars."
His discovery helps to set Andromeda apart from the Milky Way.
"It looks like our sister galaxy has led a more exciting life," Guhathakurta said. Andromeda shows signs of having at least one, if not more, major mergers with other galaxies in the relatively recent past. "In contrast the Milky Way has had a relatively quiet, quiescent last couple billion years."
Mihos and Guhathakurta presented their findings last week at the 215th meeting of the American Astronomical Society in Washington, D.C.