Sloan and the seven -- make that eight -- dwarfs (Forwarded)

Sloan Digital Sky Survey


Daniel Zucker, Cambridge University
+44 1223 337 529

Vasily Belokurov, Cambridge University
+44 1223 33 7515

David Weinberg, Scientific Spokesperson, Sloan Digital Sky Survey

Gary S. Ruderman, Public Information Officer, Sloan Digital Sky Survey

January 9, 2007

SLOAN AND THE SEVEN -- make that eight -- DWARFS

Many More Predicted

SEATTLE -- With the prospect of finding dozens of new dwarf systems in our
Local Group of galaxies, an international team of researchers from the
Sloan Digital Sky Survey (SDSS-II) has moved the count ahead with the
discovery of seven -- and perhaps eight -- new satellites of the Milky

"Cold dark matter models predict that there should be tens to hundreds
more dwarf galaxies in the Local Group than have been observed, if all
dark matter halos are lit up with stars," explains Dan Zucker, a member of
the team from Cambridge University. "In less than a year, we have used
SDSS-II data to find seven new Milky Way dwarf satellites. We've just
discovered an eighth new dwarf, but we're not sure this one is a Milky Way

"We've found almost as many new Milky Way satellites as were detected in
the previous 70 years," says Zucker's co-investigator Vasily Belokurov,
also of Cambridge.

The discovery of "A New Population of Ultra-faint Local Group Galaxies"
was announced today at the American Astronomical Society's meeting in

Dwarf galaxies contain, at most, a few million stars and they often orbit
around much larger galaxies such as the Milky Way. In its simplest form,
the leading theory of galaxy formation predicts that the Milky Way should
have a hundred or more surrounding dwarfs, but only a handful were known
before SDSS-II.

The new dwarfs have some unusual properties. "They're more like hobbits
than dwarfs," comments Belokurov, since they are smaller and fainter than
most previously known satellites. Several of the newly discovered systems
appear to be on the verge of disruption -- probably by the tidal gravity
of the Milky Way -- and the 'Ursa Major II' dwarf seems to already be in
several pieces. "They look as though they're being ground up," notes

Current theories of galaxy assembly suggest that many, perhaps all, of the
stars in the halo and thick disk of the Milky Way originated in smaller
dwarf galaxies, which were dissolved when they merged into the Milky Way

"The new dwarfs are really just the crumbs from the galactic feast," says
Zucker. "Most of the merging happened early on -- billions of years ago --
and what we're seeing here are the leftovers."

The SDSS-II is a unique resource for finding Milky Way satellites because
its deep, multi-color imaging allows detection of much fainter systems
than were previously visible. The new objects are found using
sophisticated computer algorithms that troll the digital data to find
groupings of related stars. "But the SDSS-II covers only a fifth of the
sky," notes Cambridge co-investigator Wyn Evans, "so there must be many
more dwarfs out there."

The seven new Milky Way satellites all lie in the area of sky around the
North Galactic Pole surveyed by the SDSS-II. There are two new dwarfs in
the constellation of Canes Venatici (the Hunting Dogs), one in Bootes (the
Herdsman), one in Leo (the Lion), one in Coma Berenices (Bernice's Hair),
one in Ursa Major (the Great Bear) and one in Hercules.

The eighth and newest discovery may be the most intriguing. Named Leo T,
it is about 1.4 million light years away, on the fringes of the Milky
Way's gravitational influence.

"It may be a 'free-floating' Local Group dwarf, rather than a satellite of
the Milky Way," notes team member Sergey Koposov, of the Max Planck
Institute for Astronomy in Heidelberg. In addition to its greater
distance, Leo T is distinct from the previous seven discoveries in that it
has both populations of fairly old stars (greater than five billion years
old) and comparatively young populations (less that one billion years
old). It also appears to have neutral hydrogen gas, so its star-forming
days may not be over.

Leo T could be the bellwether of a large population of faint galaxies that
reside in the Local Group but are not closely associated with either the
Milky Way or the Andromeda galaxy. Because it's too distant to be strongly
influenced by the Milky Way's tides, Leo T's low luminosity (the
equivalent of roughly 50,000 Suns) is likely intrinsic, not a consequence
of tidal stripping of loosely bound stars.

"Leo T has probably always been very faint, retaining its gas and slowly
forming stars in relative isolation," comments Mike Irwin, a discovery
team member and Cambridge University astronomer.

In combination with previously discovered systems from the SDSS-II and
other sky surveys, the large number of new dwarfs changes the complexion
of the cold dark matter theory's "missing satellite" problem. "These
discoveries bring the data and the theory closer together," comments
Zucker, though there may still be a gap between them.

Other members of the SDSS-II discovery team include Mark Wilkinson, Mike
Fellhauer, and Gerry Gilmore of Cambridge University, and Jelte De Jong
and Hans-Walter Rix of the Max Planck Institute for Astronomy.

The new finds are part of SEGUE (the Sloan Extension for Galactic
Understanding and Exploration), one of three surveys comprising SDSS-II.
"The results from SDSS-I showed us that there was a great potential for
finding new dwarf galaxies and the stars that have been ripped away from
them by the Milky Way's gravity. They were one of the major reasons we
undertook SEGUE," explains SEGUE founder Heidi Jo Newberg of Rensselaer
Polytechnic Institute. "SDSS-II is likely to turn up more of these dwarf
galaxies by the time it is done."


The Sloan Digital Sky Survey-II ( is the most ambitious
survey of the sky ever undertaken. With more than 300 astronomers and
engineers in 25 institutions around the world, the SDSS-II is continuing
to map one quarter of the entire sky, determining the position and
brightness of hundreds of millions of celestial objects, including the
measurement of distances to more than a million galaxies and quasars from
the Apache Point Observatory in New Mexico. In addition, the SEGUE (Sloan
Extension for Galactic Understanding and Exploration) will undertake the
mapping of the structure and stellar makeup of the Milky Way Galaxy. The
new Supernova Survey will repeatedly scan a 300 square degree area to
detect and measure supernovae and other variable objects.

Funding for SDSS-II has been provided by the Alfred P. Sloan Foundation,
the Participating Institutions, the National Science Foundation, the U.S.
Department of Energy, the Japanese Monbukagakusho, and the Max Planck

The SDSS is managed by the Astrophysical Research Consortium for the
Participating Institutions. The Participating Institutions are the
American Museum of Natural History, Astrophysical Institute Potsdam,
University of Basel, Cambridge University, Case Western Reserve
University, University of Chicago, Drexel University, Fermilab, the
Institute for Advanced Study, the Japan Participation Group, Johns Hopkins
University, the Joint Institute for Nuclear Astrophysics, the Kavli
Institute for Particle Astrophysics and Cosmology, the Korean Scientist
Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National
Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the
Max-Planck-Institute for Astrophysics (MPA), New Mexico State University,
Ohio State University, University of Pittsburgh, University of Portsmouth,
Princeton University, the United States Naval Observatory, and the
University of Washington.


[Image 1:]
Crumbs from the galactic feast

Researchers from the Sloan Digital Sky Survey (SDSS-II) announced the
discovery of eight new dwarf galaxies, seven of them satellites orbiting
the Milky Way. They resemble systems cannibalized by the Milky Way
billions of years ago to build up its stellar halo and thick disk. The
systems discovered by the SDSS-II in the last three years are comparable
in number to all the Milky Way satellites detected in the preceding 70
years. They help close the gap between the observed number of dwarf
satellites and theoretical predictions.

CREDIT: Vasily Belokurov, SDSS-II Collaboration and reproduced by
permission, copyright 2007, Astronomy magazine, Kalmbach Publishing Co.

[Image 2:]
One of the most intriguing finds is Leo T, about 1.4 million light years
away, on the fringes of the Milky Way's gravitational influence. Distinct
from the other finds, Leo T has old and young star populations, and its
star-forming days may not be over. In the preceding illustration, Leo T --
whose discovery was confirmed only in the last month -- would be located
roughly behind arm A of the Sagittarius stream (144 degrees Right
ascension, 17 degrees Declination).

CREDIT: Mike Irwin, the SDSS-II Collaboration


V. Belokurov
Institute of Astronomy
University of Cambridge, Madingley Rd. Cambridge CB3 0HA, UK

D. B. Zucker
Institute of Astronomy, University of Cambridge

M. J. Irwin
Institute of Astronomy, University of Cambridge

N. W. Evans
Institute of Astronomy, University of Cambridge

J. T. Kleyna
Institute for Astronomy, University of Hawaii
2680 Woodlawn Drive, Honolulu, HI 96822

E. V. Ryan-Weber
Institute of Astronomy, University of Cambridge

J. T. A. de Jong
Max Planck Institute for Astronomy, Koenigstuhl 17, 69117 Heidelberg,

S. Koposov
Max Planck Institute for Astronomy

M. I. Wilkinson
Institute of Astronomy, University of Cambridge

S. T. Hodgkin
Institute of Astronomy, University of Cambridge

G. Gilmore
Institute of Astronomy, University of Cambridge

P. Prema
Institute of Astronomy, University of Cambridge

L. Hebb
School of Physics and Astronomy, University of St. Andrews, North Haugh,
St Andrews KY16 9SS, UK

A. Begum
Institute of Astronomy, University of Cambridge

M. Fellhauer
Institute of Astronomy, University of Cambridge

P. C. Hewett
Institute of Astronomy, University of Cambridge

R. C. Kennicutt, Jr.
Institute of Astronomy, University of Cambridge

D. M. Bramich
Institute of Astronomy, University of Cambridge

S. Vidrih
Institute of Astronomy, University of Cambridge

H.-W. Rix
Max Planck Institute for Astronomy

E. F. Bell
Max Planck Institute for Astronomy

R. F. G. Wyse
The Johns Hopkins University, 3701 San Martin Drive, Baltimore, MD 21218

H. J. Newberg
Rensselaer Polytechnic Institute, Troy, NY 12180

B. Yanny
Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510

C. M. Rockosi
Lick Observatory, University of California, Santa Cruz, CA 95064

O. Y. Gnedin
Department of Astronomy, University of Michigan
830 Dennison Bldg., 500 Church St., Ann Arbor, MI 48109

D. P. Schneider
Department of Astronomy and Astrophysics, Pennsylvania State University
525 Davey Laboratory, University Park, PA 16802

J. A. Smith
Department of Physics and Astronomy, Austin Peay State University
P.O. Box 4608, Clarksville, TN 37040

T. C. Beers
Department of Physics and Astronomy, Michigan State University, East
Lansing, MI 48824

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

H. Brewington
Apache Point Observatory

J. Brinkmann
Apache Point Observatory

M. Harvanek
Apache Point Observatory

S. J. Kleinman
Subaru Telescope, 650 N. A'ohoku Place, Hilo, HI 96720

J. Krzesinski
Cracow Pedagogical University, ul. Podchorazych 2, 30-084 Cracow, Poland

D. Long
Apache Point Observatory

A. Nitta
Gemini Observatory, 670 N. A'ohoku Place, Hilo, HI 96720

S. A. Snedden
Apache Point Observatory