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Surveys Spot Ring Around the Milky Way

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Science  10 Jan 2003:
Vol. 299, Issue 5604, pp. 183
DOI: 10.1126/science.299.5604.183a

SEATTLE—Two teams of astronomers rang in the new year by announcing that our galaxy—like Saturn and a certain hobbit—possesses a grand ring of its own. The thick torus of stars, about twice as far from the center of the galaxy as our sun, probably arose after the Milky Way shredded a much smaller neighbor billions of years ago. “This is a vivid smoking gun of the disruption of a satellite galaxy,” says astronomer Bruce Margon of the Space Telescope Science Institute in Baltimore, Maryland.

Galactic jewel.

New surveys have unveiled a thick ring of hundreds of millions of stars around the outskirts of our Milky Way. The structure is about 120,000 light-years wide.

CREDIT: GERAINT F. LEWIS/UNIV. OF SYDNEY

The new ring, described here on 6 January at a meeting of the American Astronomical Society, is a galaxy-spanning structure consisting of hundreds of millions of stars. It had remained hidden because it lies in the plane of the Milky Way's disk, which contains our sun and most of the galaxy's stars, gas, and dust. Two sensitive surveys of broad patches of the sky have now peered through that clutter to expose a procession of stars beyond the disk, embracing the galaxy in a circle about 120,000 light-years wide.

Astronomers had seen much smaller arcs and ribbons of stars coursing through our galaxy's vast outer halo, a sparsely populated sphere of old stars. Researchers think these stellar streams are the remnants of dwarf galaxies and globular clusters that dissolved in the Milky Way's powerful gravitational field (Science, 3 January, p. 62). By using a 3.5-meter telescope at the Sloan Digital Sky Survey's (SDSS's) Apache Point Observatory in Sunspot, New Mexico, astronomers have now identified a band of stars wrapping one-sixth of the way around the galaxy.

The smooth distribution of the stars suggests that a satellite galaxy merged with the Milky Way as long ago as 10 billion years, giving the stars ample time to spread out. Independently, European and Australian astronomers using the 2.5-meter Isaac Newton Telescope at La Palma, Canary Islands, spotted the torus in two other directions, strengthening the suspicion that it girdles the galaxy.

Astronomers have floated other ideas about how the ring arose. Its near-perfect alignment with the Milky Way's disk suggests that the stars in the ring might have spread out from the galaxy itself, says astronomer Annette Ferguson of the University of Groningen, the Netherlands, a member of the Canary Islands team. But SDSS astronomer Heidi Newberg of Rensselaer Polytechnic Institute in Troy, New York, notes that stars in the ring move through space at similar speeds—whereas stars puffed up from the disk would have more-scattered velocities. “We have to assume that this is a group of stars coherently moving together” and with a common birthplace, Newberg says.

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