Astronomers find evidence of galaxy torn apart by Milky Way

Sunday, January 12, 2003

SEATTLE -- Like a celestial bully, the Milky Way may have ripped apart a smaller galaxy billions of years ago and scattered its stars into a faint surrounding ring.

A new survey scanning the outskirts of the Milky Way has found a belt of stars that are different in chemistry and in motion from stars within the galaxy, suggesting they are the remnants of a galactic collision that may have occurred 10 billion years ago.

The ring, some 120,000 light-years across, is shaped like a doughnut with the Milky Way in its center.

"This is something new that we had not expected to find," said Heidi Jo Newberg of Rensselaer Polytechnic Institute. She and Brian Yanny of the Fermi National Accelerator Laboratory discovered the ring during the Sloan Digital Sky Survey, a project to plot the position and brightness of 100 million celestial objects.

A European team, using different instruments, has confirmed the existence of the ring.

Newberg and Yanny presented their discovery last week at the national meeting of the American Astronomical Society.

Bruce Margon of the Space Telescope Science Institute, said the new finding by Yanny and Newberg "is a vivid smoking-gun evidence of the disruption of galaxy" and said it was "embarrassing" that the structure had not been found earlier.

"It has been accepted folklore that big galaxies tear apart and eat smaller galaxies," said Margon, and the new evidence shows that an entire small galaxy was once gobbled up by the Milky Way.

Yanny said that the ring has 100 million to a half-billion stars circling the Milky Way in an orbital period of tens of thousands of years.

Newberg said the stars in the ring show that they are moving "in a very similar fashion to way that planets orbit the sun."

Other evidence shows clearly, she said, "that we are seeing pieces of another galaxy coming in and being ripped apart and incorporated by our galaxy."

Earlier studies have shown that Milky Way has bullied other smaller galaxies. A structure called the Sagittarius tidal stream, which circles the Milky Way at an angle, is thought to be the remains of a small galaxy. And astronomers have found evidence that the Milky Way is beginning the process of cannibalizing the Magellanic Cloud, a nearby galaxy expected to fall into the Milky Way in a few billion years.

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