Albuquerque, N.M. -- 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
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 meeting in Albuquerque,
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
``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
``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
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.
Fig. 1: Color-coded map of the distribution of stars
emerging from the star cluster Palomar 5 (white blob).
The two long tidal tails (orange) contain 1.3 times the mass
of the cluster and delineate its orbit around the Milky Way
Fig. 1 in PDF format without orbit
Fig. 1 in PDF format with orbit
Fig. 1 in PDF format with orbit and labels
Fig. 2: The orbit (red line) of Palomar 5 in the halo of
our Milky Way as reconstructed from the tidal tails and the known
position, distance and radial velocity of the cluster.
The image used to illustrate the Milky Way Galaxy is courtesy
of the Hubble Heritage project (STScI/NASA).
Fig. 2 in PDF format