January 11, 2006

The Sloan Digital Sky Survey Reveals A New Milky Way Neighbor

SDSS -- A huge but very faint structure, containing hundreds of thousands of stars spread over an area nearly 5,000 times the size of a full moon, has been discovered and mapped by astronomers of the Sloan Digital Sky Survey (SDSS-II).

At an estimated distance of 30,000 light years (10 kiloparsecs) from Earth, the structure lies well within the confines of the Milky Way Galaxy. However, it does not follow any of Milky Way's three main components: a flattened disk of stars in which the sun resides, a bulge of stars at the center of the Galaxy and an extended, roughly spherical, stellar halo. Instead, the researchers believe that the most likely interpretation of the new structure is a dwarf galaxy that is merging into the Milky Way.

The new dwarf galaxy is found toward the constellation Virgo.

"Some of the stars in this Milky Way companion have been seen with telescopes for centuries,'" explained Princeton University graduate student Mario Juric, principal author of the findings describing what may well be our closest galactic neighbor. "But because the galaxy is so close, its stars are spread over a huge swath of the sky, and they always used to be lost in the sea of more numerous Milky Way stars. This galaxy is so big, we couldn't see it before."

The result was presented today in a session on The Milky Way at the American Astronomical Society meeting in Washington, D.C.

The discovery was made possible by the unprecedented depth and photometric accuracy of the SDSS, which to date has imaged roughly one-quarter of the northern sky. "We used the SDSS data to measure distances to 48 million stars and build a 3-d map of the Milky Way," explained Zeljko Ivezic of the University of Washington, a co-author of the study. Details of this "photometric parallax" method - using the colors and apparent brightnesses of stars to infer their distances -- are explained in the paper "Milky Way Tomography" submitted to The Astrophysical Journal. It is available in preprint form at http://www.arxiv.org/abs/astro-ph/0510520.

"It's like looking at the Milky Way with a pair of 3-d glasses," said Princeton University co-author Robert Lupton. "This structure that used to be lost in the background suddenly snapped into view."

The new result is reminiscent of the 1994 discovery of the Sagittarius dwarf galaxy, by Rodrigo Ibata and collaborators from Cambridge University. They used photographic images of the sky to identify an excess of stars on the far side of the Milky Way, some 75,000 lightyears from Earth. The Sagittarius dwarf is slowly dissolving, trailing streams of stars behind it as it orbits the Milky Way and sinks into the Galactic disk.

In the last decade a new generation of sky surveys using large digital cameras identified a number of streams and lumps of stars in the outer Milky Way. Some of these lumps are probably new Milky Way companions; others may be shreds of the Sagittarius dwarf or of other dissolving dwarf galaxies. Earlier SDSS discoveries include an apparent ring of stars encircling the Milky Way disk that may be the remnant of another disrupted galaxy; and the Ursa Major dwarf, the faintest known neighbor of the Milky Way.

The first hints of an unusually high density of stars in the direction of Virgo were made in 2001 by the QUEST survey, which used a 1-meter telescope in Venezuela to study a class of variable stars called RR Lyrae variables.

"We found a clump of 5 RR Lyrae stars, and speculated that they were they belonged to a small galaxy being cannibalized by the Milky Way," explained Kathy Vivas of the Centro de Investigaciones de Astronomia in Venezuela, who (as a Yale graduate student) was the author of the QUEST discovery paper. "In light of the new SDSS results," Vivas added, "it appears that the stellar stream we detected is itself part of the larger structure identified by Juric and collaborators."

In the January 10, 2006 Astrophysical Journal Letter (astro-ph/0510589), the QUEST team led by Universidad de Chile graduate student Sonia Duffau presented further evidence for this interpretation by measuring the motions and chemical compositions of stars in the region.

"With so much irregular structure in the outer Galaxy, it looks as though the Milky Way is still growing, by cannibalizing smaller galaxies that fall into it," said Juric.

Another group of SDSS astronomers used the data to find the two faintest-known companions of the Andromeda Galaxy, the closest giant spiral galaxy similar in size to the Milky Way. Daniel Zucker of the Max Planck Institute of Astronomy in Heidelberg and Cambridge University's Institute of Astronomy led that team.

"These new Andromeda companions, alongside the new Milky Way neighbors, suggest that faint satellite galaxies may be plentiful in the Local Group," said Zucker.

While the SDSS was originally designed to study the distant universe, its wide area, high precision maps of faint stars have made it an invaluable tool for studying the Milky Way and its immediate neighborhood. The 3-d map created by Juric and his collaborators also provides strong new constraints on the shape and extent of the Milky Way's disk and stellar halo.

Another Princeton graduate student, Nick Bond, is using the subtle motions of stars detected during the five-year span of the SDSS-I observations to limit the amount of dark matter in the solar neighborhood.

University of Washington graduate student Jillian Meyer is mapping the distribution of interstellar dust, by carefully studying the colors of stars found in both the SDSS and the infrared 2MASS survey.

Building on these many successes, the SEGUE project (Sloan Extension for Galactic Understanding and Exploration) will use the SDSS-II telescope, its 120-megapixel digital camera and its 640-fiber optical spectrograph to carry out detailed studies of the structure and chemical evolution of the Milky Way.

SEGUE is one of three components of SDSS-II, the three-year extension of the Sloan Survey continuing through mid-2008.

Fermilab scientist Brian Yanny, one of the SEGUE team leaders, is excited at the prospect of examining its just completed, first season of observations.

"The SDSS has already told us surprising things about the Milky Way, but the most exciting discoveries should lie just ahead."


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