UMass Amherst Astronomer on Team that Finds Milky Way Consuming a Neighboring Galaxy

AMHERST, Mass. – Thousands of stars stripped from the nearby Sagittarius dwarf galaxy are streaming through our vicinity of the Milky Way galaxy, according to a team of astronomers from the University of Massachusetts Amherst and the University of Virginia. Using new data from the Two-Micron All Sky Survey (2MASS), the astronomers have created a new view of the universe that shows the Milky Way is consuming one of its neighbors in a dramatic display of ongoing galactic cannibalism.

2MASS is a major project to survey the sky in infrared light led by UMass. This latest study, to be published in the Dec. 20 issue of the Astrophysical Journal, is the first to map the full extent of the Sagittarius galaxy and show in visually vivid detail how its debris wraps around, and passes through the Milky Way. Sagittarius is 10,000 times smaller in mass than the Milky Way and is getting stretched out, torn apart, and gobbled up by the larger galaxy.

"After slow, continuous gnawing by the Milky Way, Sagittarius has been whittled down to the point that it cannot hold itself together much longer," says 2MASS team member and study co-author, Martin D. Weinberg, professor of astronomy at UMass Amherst. "We are seeing Sagittarius at the very end of its life as an intact system."

"It’s clear who’s the bully in the interaction," says Steven Majewski, professor of astronomy at the University of Virginia and lead author on the paper describing the results. In model images made to show the interaction in 3-D, the Milky Way appears as a flattened disk with spiral arms, while Sagittarius is visible as a long flourish of stars swirling first under and then over and onto the Milky Way disk, he says.

"If people had infrared-sensitive eyes, the entrails of Sagittarius would be a prominent fixture sweeping across our sky," Majewski says. "But at human, visual wavelengths, they become buried among countless intervening stars and obscuring dust from our own Milky Way. The great expanse of the Sagittarius system has been hidden from view."

The new findings will also help astronomers measure the total mass of the Milky Way and Sagittarius galaxies, and probe the quantity and distribution of the invisible and mysterious dark matter in these systems. "The shape of the Sagittarius debris trail shows us that the Milky Way’s unseen dark matter is in a spherical distribution, a result that is quite unexpected," says Weinberg.

2MASS was a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology funded by the National Aeronautics and Space Administration, and the National Science Foundation. Additional funding for the Sagittarius study with 2MASS came from the David and Lucile Packard Foundation.

Martin D. Weinberg can be reached at 413/545-3821 or

Steven Majewski can be reached at 434/924-4893 or

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