The Sloan Digital Sky Survey Reveals A New Milky Way Neighbor (Forwarded)

Sloan Digital Sky Survey

CONTACTS:

Robert Lupton
Princeton University
At AAS: 609 233 2011

David Weinberg
Scientific Spokesperson, The Sloan Digital Sky Survey
At AAS: 614-406-6243

Gary S. Ruderman
Public Information Officer, The Sloan Digital Sky Survey
312-320-4794

January 9, 2006

The Sloan Digital Sky Survey Reveals A New Milky Way Neighbor

WASHINGTON, DC -- 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 light
years 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."

AUTHORS:

Mario Juric
Princeton University Observatory, Princeton, NJ 08544

Zeljko Ivezi
University of Washington, Dept. of Astronomy, Box 351580, Seattle, WA 98195

Alyson Brooks, University of Washington

Robert H. Lupton, Princeton University Observatory

David Schlegel, Princeton University Observatory

Douglas Finkbeiner, Princeton University Observatory

Nikhil Padmanabhan
Princeton University, Dept. of Physics, Princeton, NJ 08544

Nicholas Bond, Princeton University Observatory

Constance M. Rockosi, University of Washington

Gillian R. Knapp, Princeton University Observatory

James E. Gunn, Princeton University Observatory

Takahiro Sumi, Princeton University Observatory

Donald Schneider
Department of Astronomy and Astrophysics, Pennsylvania State University,
University Park, PA 16802

J.C. Barentine
Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349, U.S.A.

Howard J. Brewington, Apache Point Observatory

J. Brinkmann, Apache Point Observatory

Masataka Fukugita
6 University of Tokyo, Institute for Cosmic Ray Research

Michael Harvanek, Apache Point Observatory

S.J. Kleinman, Apache Point Observatory

Jurek Krzesinski
Apache Point Observatory and Mt. Suhora Observatory, Cracow Pedagogical
University, ul. Podchorazych 2, 30-084 Cracow, Poland

Dan Long, Apache Point Observatory

Eric H. Neilsen, Jr.
Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510,
U.S.A.

Atsuko Nitta, Apache Point Observatory

Stephanie A. Snedden, Apache Point Observatory

Donald G. York, Department of Astronomy and Astrophysics, The University
of Chicago, Chicago, IL 60037 USA

IMAGE CAPTION:
[http://www.sdss.org/news/releases/virgoOverdensity.png (419KB)]
The figure by Mario Juric of Princeton university shows the counts of
faint blue stars selected from a narrow magnitude and color range, and
corresponding to a distance from Earth of about 10 kpc. The map shows
the view from far away above the galactic plane.

According to the standard Milky Way models, the top and bottom halves
should be symmetric with respect to the horizontal line in the middle,
because the Milky Way is believed to be symmetric around its rotation
axis that passes through the Galactic Center and is perpendicular to
this map.

The discovery of the large overdensity of stars at the longitudes of
about 300 degrees and latitudes of about 60 degrees breaks the expected
symmetry and is at odds with the standard models.

The dashed line shows the position of the plane containing debris from
the Sagittarius dwarf galaxy that is being cannibalized by the Milky
Way. While its proximity to the new structure suggests that perhaps the
two may be related, the known Sagittarius debris is located about four
times further away.

(The map is based on upcoming SDSS-II Data Release 5, and is shown in
Lambert projection of galactic coordinates: radial rays are lines of
constant longitude, circles are lines of constant latitude, the North
Galactic Pole is in the center, and the Galactic Center is towards the
left. The counts are shown on a logarithmic stretch, with a dynamic
range of 10 increasing from blue to red).

CREDIT - M. Juric/SDSS-II Collaboration