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Cannibal Galaxies Gobble the Little Guys

Galaxies are big cannibals that constantly gobble up smallergalaxies nearby.

Scientists have just discovered new signs of this cosmic carnagein nearby galaxies, including our closest neighbor, Andromeda.

When wimpy galaxies pass near behemoths like the MilkyWay, the unfortunate weaklings are slowly swallowed up, their starsintegrated into the parent. But they leave behind evidence of their demise inthe form of so-called tidal streams, which are trails of stars that were rippedoff the small galaxies by gravitational forces.

The tidal streams trace the path the mini galaxies followedas they orbited around their parent before they were swallowed up. Scientistscan use these orbits to measure how much the larger galaxies weigh and howtheir mass is spread out, and can even learn more about how the galaxies wereformed and evolved.

"You can see these very complex systems of shells andplumes of tidal debris that mark the past accretion history of thegalaxy," said astronomer Chris Mihos of Case Western Reserve University in Cleveland, Ohio.

Mihos recently found signs of tidal tails around some of thegalaxies in the Virgo Cluster, a relatively nearby collection of galaxies about50 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 theform of tidal streams around the Andromeda galaxy, only 2.5 million light-yearsaway.

"The tidal stream gives you a window in time duringwhich the events happened: the last couple billion years," Guhathakurtasaid. "If it was earlier, it's unlikely we'd still see the stars."

His discovery helps to set Andromeda apart from the MilkyWay.

"It looks like our sistergalaxy has led a more exciting life," Guhathakurta said. Andromedashows signs of having at least one, if not more, major mergers with othergalaxies in the relatively recent past. "In contrast the Milky Way has hada relatively quiet, quiescent last couple billion years."

Mihos and Guhathakurta presented their findings last week atthe 215th meeting of the American Astronomical Society in Washington, D.C.

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