Sky Survey Unveils Star Cluster Shredded By The Milky Way

Albuquerque, New Mexico, USA, and Heidelberg, Germany — A team of
astronomers from the Sloan Digital Sky Survey (SDSS) collaboration
has discovered a spectacular stream of stellar debris emanating from
a star cluster that is being torn apart by the Milky Way.

Dr. Michael Odenkirchen and Dr. Eva Grebel from the Max Planck
Institute for Astronomy (MPIA) in Heidelberg, Germany, are presenting
these findings today at the American Astronomical Society (AAS)
meeting in Albuquerque, New Mexico. The detection of this stream,
the first of its kind, supports theorists’ view that star clusters
get destroyed by the tidal forces of the Milky Way. Researchers say
such extended streams of tidal debris provide a new way to determine
the mass distribution of the dark matter halo of our Galaxy.

The stars in the newly discovered stream are being torn from an
ancient globular cluster named Palomar 5, which is located in the
outer part of our Galaxy 75,000 light years away from the Sun. While
typical globular clusters are massive, luminous concentrations of
some hundred thousand stars, Palomar 5 by comparison looks faint
and diffuse and contains only about ten thousand stars. This led
astronomers to suspect that Palomar 5 might be a likely victim of
the disruptive tides of the Milky Way. These “tides” arise because
the Milky Way’s gravitational pull is stronger on the cluster’s near
side than on the far side, thus tearing the cluster apart. However,
the telltale debris from the disruption was difficult to find since
it is hidden in a sea of foreground and background objects.

Using data from the SDSS and a special filtering technique,
Odenkirchen and his collaborators have succeeded in making the
stream of debris from Palomar 5 directly visible. “The excellent
homogeneity, resolution, depth, and multi-color information of the
SDSS observations have allowed us to separate faint former members
of Palomar 5 from contaminating field stars and background galaxies,”
says Odenkirchen, who is a postdoctoral researcher at the MPIA.

The SDSS is an international project that is creating a deep map of
one quarter of the sky in five colors. The SDSS records objects up
to 10 million times fainter than the faintest stars visible with
the naked eye. The observations are carried out with a special
wide-field camera on a dedicated 2.5-meter telescope at Apache Point
Observatory, New Mexico. Team member Dr. Connie Rockosi of the
University of Washington was one of the builders of the camera.

First direct evidence for the tidal disruption of Palomar 5 emerged
two years ago from SDSS commissioning data that happened to include
Palomar 5. Odenkirchen and collaborators were amazed to recover the
characteristic S-shape signature of tidal debris from these data.
“This was the first time that tidal tails of a star cluster were
seen with convincing clarity”, says Grebel, an astronomer who leads
the Galactic structure group at MPIA.

Meanwhile the SDSS has scanned a much larger region on the sky.
Analyzing the new data the researchers found that the two tails
emanating from Palomar 5 extend over an arc of ten degrees on the
sky. This vast area corresponds to 20 times the diameter of the
full moon on the sky or to a length of 13,000 light years in space.
“Remarkably, we now find more mass in the tails than in the
remaining cluster. We expect to detect the stream over an even
larger area as the survey progresses,” Odenkirchen said.

The tails of Palomar 5 delineate the orbital path of this cluster
and thus provide a unique opportunity to determine its motion
around the Milky Way. “The motions of objects orbiting the Galactic
halo are still poorly known. It normally takes decades to measure
even only the instantaneous displacement of a globular cluster on
the sky,” Grebel points out. “Finding additional coherent streams
that extend over large portions of the sky we would be able to
reconstruct Galactic orbits independent of a specific Galactic
model,” says the MPIA’s Dr. Walter Dehnen, who carried out extensive
numerical simulations on the disruption of Palomar 5. The researchers
expect that the geometry and the velocities of those tidal streams
will become important tools for determining the mass of the dark
matter halo of the Milky Way.

Together with the so-called Sagittarius stream, which emerges from
a dwarf galaxy that is currently being accreted by the Milky Way,
there are now two different examples of extended stream-like
structures in the Galactic halo. Computer simulations suggest that
globular clusters were much more numerous in the early days of the
Milky Way, and that many of them have already been shredded by
Galactic tides. As the survey proceeds the SDSS researchers will be
able to test this prediction by searching for signs of tidal mass
loss around other globular clusters. “The SDSS data base will
ultimately allow us to estimate the total number of such streams,”
says Professor Hans-Walter Rix, director of the MPIA. “This will
clarify the role of tidal disruption in the build-up of the Galactic
halo and provide a crucial test for galaxy formation models.”

The researchers participating in this work are Michael Odenkirchen,
Eva Grebel, Walter Dehnen, and Hans-Walter Rix from the Max Planck
Institute for Astronomy, Connie Rockosi of the University of
Washington, Brian Yanny from Fermilab, and Heidi Newberg from the
Rensselaer Polytechnic Institute.

The SDSS is a joint project of The University of Chicago, Fermilab,
the Institute for Advanced Study, the Japan Participation Group,
The Johns Hopkins University, Los Alamos National Laboratory, the
Max-Planck Institute for Astronomy (MPIA), the Max-Planck Institute
for Astrophysics (MPA), New Mexico State University, Princeton
University, the United States Naval Observatory, the University of
Pittsburgh, and the University of Washington.

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

[NOTE: Images supporting this release are available at
http://www.mpia-hd.mpg.de/Public/Aktuelles/PR/2002/PR020603/PR_020503_en.html ]