Galactic cannibalism

The Milky Way has been at it again. Astronomers have found evidence that our home galaxy is tearing apart and swallowing a nearby collection of stars—most likely the remains of a dwarf galaxy. The galactic violence would be the latest confirmed act of cannibalism by the Milky Way (SN: 4/22/00, p. 261: Available to subscribers at Milky Way feasts on its neighbors). Galaxies commonly grow by eating each other.

Some 30,000 light-years from Earth, the collection of stars lies within the Milky Way but differs in velocity and shape from that of other known galaxy components. Containing several hundred thousand stars, the group spans a swath of sky larger than any galaxy beyond the Milky Way.

Astronomers found the group by combing through distance measurements of 48 million stars recorded by the Sloan Digital Sky Survey. The data enabled the team to build a three-dimensional map of the Milky Way in which faint star groupings “snapped into view,” says Robert Lupton of Princeton University. He reported the findings in January at a meeting of the American Astronomical Society in Washington, D.C.

Another team, reporting in the Jan. 10 Astrophysical Journal Letters, already had evidence that a subset of stars in the same region moves in concert and is probably the remains of a dwarf galaxy. But the Sloan study revealed the breadth of the newfound collection of stars.

Because the former galaxy lies considerably closer to the center of the Milky Way than do any other known dwarf-galaxy remains, “it is likely to have been chewed up more by the interactions with the Milky Way’s gravitational field,” says Heather Morrison of Case Western Reserve University in Cleveland.

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