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Researchers Say New Star Structures Found in the Milky Way Alter Galactic Model

January 9, 2002

Researchers Say New Star Structures Found in the Milky Way Alter Galactic Model

Troy, N.Y. — Heidi Jo Newberg, associate professor of physics at Rensselaer Polytechnic Institute, and Brian Yanny, an astrophysicist at Fermi National Accelerator Laboratory, who are leading a team of researchers with the Sloan Digital Sky Survey (SDSS), announced today they have identified new star structures in the halo of the Milky Way that could alter the standard model of the galaxy. The research also has implications for how the Milky Way was formed.

The research, presented at a meeting of the American Astronomical Society in Washington D. C., may be a first step in the development of complete galactic models for the halo. The star streams were identified from positions, colors, and brightnesses of five million stars detected in the Sloan Digital Sky Survey. The SDSS is an international scientific collaboration which is cataloging the heavens to an unprecedented depth, area, and accuracy.

Newberg and Yanny say the results yielded by the SDSS database provide a “deeper, more global picture of the Milky Way’s stellar system.”

An unexpectedly large number of blue stars have been found within 20 degrees of the galactic plane, say Newberg and Yanny. These stars could be part of a disrupted dwarf galaxy, or a disk-like distribution of stars that is puffier than accepted models of stellar disks in the galaxy, and flatter than the spherical distribution in the halo.

“The clumpiness of the stellar distribution in the Milky Way halo suggests that our galactic model needs to be reconsidered,” says Newberg. “Although we originally set out to measure properties of a smooth halo, we now find it difficult to determine which, if any, of the structures of the halo belong to that population.”

“Stars in the halo appear to be grouped into distinct streams in the sky,” says Yanny. “A careful look at the stellar properties shows that they come from yet unidentified parent populations, perhaps other dwarf galaxies which have long since been torn apart.”

Newberg and Yanny are the principal authors, along with 17 SDSS researchers, of a paper to be published by The Astrophysical Journal.

Newberg says the findings are significant because they have an impact on several active fields of astronomical research, including: galactic structure, evolution of the Milky Way, the distribution of mass in the galaxy, and galaxy formation in the early Universe.

Newberg and Yanny presented plots of color versus brightness for stars in two previously discovered, tidally disrupted structures. The distribution of stars in these plots, which indicate stellar age and metallicity, are consistent with those of the Sagittarius dwarf galaxy, but the stars are spread across 110 square degrees of sky, 75 degrees away from the center of the Sagittarius dwarf.

Newberg and Yanny have identified at least five additional overdensities of stars in the galactic halo. Four of these may be pieces of the same halo structure, which would cover a region of the sky at least 40 degrees across, at a distance of 11 kpc (36,000 light years) from the sun which is 18 kpc, or 60,000 light years from the center of the galaxy. For reference, the Sun is 25,000 light years from the center of the galaxy.

“It is striking that in this direction in the sky all the stars appear to be at the same distance,” said Connie Rockosi, a researcher at the University of Washington, who initially found the group of blue stars. “This suggests that our galaxy might be encircled by a narrow ring of stars, possibly the result of a dwarf satellite galaxy disruption.”

The paper, titled “Halo Streams and Milky Way Components from the Sloan Digital Sky Survey,” can be found at

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