Daniel Zucker, Max Planck Institute for Astronomy,
Eric Bell, Max Planck Institute for Astronomy,
(Zucker and Bell will be attending the AMERICAN ASTRONOMICAL SOCIETY meeting in Atlanta and can be reached immediately following their panel presentation through the AAS Press Room at the Hyatt Regency Hotel -- 404-577-1234, fax 404-588-4137).
Michael A. Strauss, SDSS Scientific Spokesman, (609) 258-3808,
Gary S. Ruderman, SDSS Public Information Officer, (312)-320-4794,

ATLANTA -- A international team of astronomers from the Sloan Digital Sky Survey collaboration has discovered a giant clump of stars near the Andromeda Galaxy that could be a previously unknown satellite galaxy of Andromeda or could be the last remnants of a galaxy torn apart by Andromeda's tidal forces.

The clump of stars, named Andromeda NE by the researchers for its location to the northeast of the Andromeda Galaxy, is enormous. It covers a larger area on the sky than the full Moon. If added together the total light from the stars in Andromeda NE would rival many nearby galaxies in brightness. Yet because these stars are so spread out, Andromeda NE appears 10 times dimmer than the faintest known galaxy.

(Click to enlarge)
This composite illustration shows the location of Andromeda NE (arrow), the complicated stellar structures in the halo of the Andromeda Galaxy (inset image to scale) and a scaled photo of the moon as a size reference (to the right). The colors in the image reflect the relative colors of stars that are predominantly red giants at the distance of the Andromeda Galaxy. Bluer and whiter colors generally indicate lower metallicity (elemental abundances) and yellow and red stars indicate increasing elemental abundances. (Except for Andromeda NE, most of the major structures shown here were also seen by Ferguson et al as noted in Astronomical Journal, vol. 124, p. 1452, Sept. 2002, although the representation of SDSS data is more informative.) SOURCES: Sloan Digital Sky Survey; Inset of M31 from Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF; Photo of full moon from Lick Observatory.
"This helps to explain why it remained undiscovered," said Daniel Zucker, one of the Sloan Digital Sky Survey (SDSS) researchers. "The quality of the SDSS data allowed us to find this elusive object close to Andromeda, one of the best-studied galaxies in the sky," added team member Eva Grebel of the University of Basel. More than 2 million light years away, the Andromeda Galaxy or M31 is the nearest spiral galaxy, and is one of a handful of galaxies (besides our own Milky Way) that can be seen with the naked eye.

Zucker and Eric Bell from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany presented these findings today (January 5, 2004) at the American Astronomical Society meeting in Atlanta.

While analyzing data from an SDSS scan of Andromeda, Zucker and his team used special filtering techniques to select objects with specific colors and brightness typical of Andromeda's stars. When the SDSS team mapped the distribution of these stars they detected a number of features previously noted by other astronomers. But SDSS researchers didn't know what to make of a large concentration of stars - what they described at first as a giant, ghostly shape -- that appeared to the northeast of Andromeda.

"At first we thought it was some sort of artifact, some kind of problem with our filtering technique," says Zucker. "But test after test showed that it was really there."

Having determined that the stellar structure was real, the SDSS team members from MPIA, the University of Washington in Seattle, the University of Basel in Switzerland and New Mexico State University in Las Cruces worked to find out what it was.

"One of the most important questions about Andromeda NE is its distance. Although our findings don't allow us to measure this precisely, our data indicate that it is roughly at the same distance as the Andromeda Galaxy," said Bell.

That means that the clump of stars is gravitationally bound to the Andromeda Galaxy, in orbit around its larger neighbor and possibly is in the process of being torn apart by Andromeda's tidal forces. These forces arise because Andromeda's gravitational pull is stronger on the near side of Andromeda NE than on the far side, pulling the stars away from each other. "For most small companion galaxies it is ultimately only a matter of time until they are shredded by the tides of the parent galaxy," explained Hans-Walter Rix, director of MPIA.

Over the past decade astronomers have found increasing evidence that the distant, outer reaches of normal spiral galaxies like Andromeda and the Milky Way are not quiet backwaters, but rather arenas of ongoing galaxy disruption. As satellite galaxies are ripped apart by tidal forces, they lose stars in great streams along their orbital paths. Researchers have detected such stellar streams around both the Milky Way and Andromeda, suggesting that this kind of galactic cannibalism is commonplace.

"SDSS data have been used to study the destruction of satellites by our galaxy, and now they are adding to our knowledge of Andromeda's eating habits," observed Alexei Kniazev, an SDSS collaboration member and a researcher at MPIA.

In 2003, a team of SDSS scientists found what is believed to be the remnants of a galaxy pulled apart by the Milky Way. The SDSS's ability to do three-dimensional mapping revealed that the concentration of stars of a certain color and brightness were actually parts of a separate structure outside of the Milky Way.

So far the exact nature of Andromeda NE remains a mystery. It could be a satellite galaxy of Andromeda, perhaps one so stretched out by Andromeda's tides that it has become the enormous, remarkably diffuse structure seen today. Or it could be part of a giant stream of stars, all that remains of a satellite that was completely disrupted by Andromeda's tidal forces in the distant past. "But we'll need more data to determine which it is," said Zucker.


Daniel Zucker, Max Planck Institute for Astronomy, Konigstuhl 17, D-69117 Heidelberg, Germany,

Alexei Kniazev, Max Planck Institute for Astronomy,

Eric Bell, Max Planck Institute for Astronomy,

David Martinez-Delgado, Max Planck Institute for Astronomy,

Hans-Walter Rix, Max Planck Institute for Astronomy,

Eva Grebel, Astronomical Institute of the University of Basel, Venusstrasse 7, CH-4102 Basel, Switzerland,

Connie Rockosi, University of Washington, Department of Astronomy P.O. Box 351580, University of Washington, Seattle, WA, 98195

Jon Holtzman, Astronomy Department, New Mexico State University, Box 30001, Department 4500, Las Cruces, NM 88003.

Rene Walterbos, New Mexico State University, .

Zeljko Ivezic, Princeton University Observatory, Princeton, NJ 08544

Jonathan Brinkmann, Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349 .

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