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The Sloan Digital Sky Survey turns its eye on the Milky Way (Forwarded)



 
 
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Old January 14th 06, 05:27 AM posted to sci.astro
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Default The Sloan Digital Sky Survey turns its eye on the Milky Way (Forwarded)

Sloan Digital Sky Survey

CONTACTS:
Brian Yanny
Fermi National Accelerator Laboratory (Fermilab)
630-840-4413

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 11, 2006

THE SLOAN DIGITAL SKY SURVEY TURNS ITS EYE ON THE GALAXY

A CLOSER LOOK AT THE MILKY WAY

Washington, DC -- An amazing array of new stellar findings were released
today in a special session "Galactic Astronomy and The Sloan Digital Sky
Survey."

The tilted, egg-shaped Milky Way stellar halo, an explosion in the
number of "pristine," ancient stars, variations in the age and chemical
make-up of open cluster stars, 'high velocity' stars in the Galactic
halo, and the search for other rare and interesting objects were part of
the earliest findings from SEGUE, the Sloan Extension for Galactic
Understanding and Exploration.

The Sloan Digital Sky Survey (SDSS-II) presentation was made today at
the American Astronomical Association's winter meeting in Washington,
DC. SEGUE is one of three observing programs that collectively make up
SDSS-II, an extension through mid-2008 and an expansion of the original
Sloan Digital Sky Survey (SDSS).

"SEGUE will provide an extraordinarily rich data set for studying the
global structure and stellar population content of the Milky Way,"
explained Connie Rockosi, an astronomer from the University of
California's Lick Observatory and a co-leader of the SEGUE team.

SEGUE will extend the five-band imaging survey of the precursor SDSS to
cover an additional 3500 square degrees, reaching into the plane of the
Galactic disk. It will also obtain spectroscopy for 240,000 stars
selected from the images.

"The maps of the stellar distribution and measurements of stellar
motions and chemical abundances will collectively tell us an enormous
amount about the formation, evolution, and merger history of the Milky
Way," added Rockosi.

The Sloan Digital Sky Survey (www.sdss.org) is the most ambitious
astronomical survey project ever undertaken. The survey will map
one-quarter of the entire sky, determining the positions and absolute
brightnesses of more than 100 million celestial objects. It will also
measure the distances to more than a million galaxies and quasars from
the Apache Point Observatory in New Mexico.

A more complex stellar halo

In a presentation entitled In Search of the Stellar Spheroid from SDSS
and SEGUE, Heidi Newberg of the Rensselaer Polytechnic Institute, one of
the initiators of the SEGUE project, discussed the picture of the Milky
Way's extended stellar halo that is emerging from SDSS maps.

"The stellar halo appears to be much more complex than the smooth,
symmetric system envisioned in early models of the Galaxy. We find many
lumps and streams of tidal debris, strong asymmetry, and even a hint
that the stellar halo is not centered on the center of the Galactic
disk." Tidal debris consists of stars that have been torn from small
galaxies by the tidal gravity of the Milky Way.

In another presentation -- High-latitude Structure in the Milky Way Halo
-- SEGUE co-leader Brian Yanny of the Fermi National Accelerator
Laboratory (Fermilab) presented some of the survey's early results on
structure in the Galactic halo. Drawing on photometric distance
estimates for millions of stars from precise multi-color imaging, and
velocity and chemical abundance measurements from medium-resolution
spectroscopy of many thousands of stars, SEGUE reveals several
intriguing coherent structures at distances ranging from 5 kiloparesecs
(kpcs) to 50 kpc from the Sun. (One kiloparsec is about 3000 light years.)

"Pristine" ancient stars

The nuclei of most hydrogen and helium atoms were forged in the hot
early universe, but heavier elements, which astronomers generally refer
to as "metals," are made in the interiors of massive stars, then
expelled into the surrounding gas when the stars reach the ends of their
lives.

"Low metallicity" stars, with only the tiniest traces of these heavier
elements, are the relics of the earliest generations of star formation
in the galaxy, forming from gas that had been only minimally polluted by
their predecessors.

In his presentation on Low Metallicity Stars in SDSS and SEGUE, SDSS-II
member Timothy Beers of Michigan State University (part of the Joint
Institute for Nuclear Astrophysics) noted that previous surveys for
stars with low abundances of heavy metals, such as iron, identified
several thousand "very metal poor" (VMP) stars, with metal abundance
less than 1/100th of the Sun's. These surveys found only a few hundred
"extremely metal poor" (EMP) stars below 1/1000th of the Sun's iron
abundance. Only a handful of the rare, "ultra metal poor" (UMP) stars
below 1/10,000th of the Sun's abundance have ever been found.

But SEGUE is poised to radically transform this situation.

"Even though the first SDSS was primarily aimed at the observation of
galaxies," Beers explained, "the spectra of tens of thousands of
calibration stars' revealed more than 2,000 stars with metal abundances
less than 1/100th of the Sun's. This is a prodigious yield for a class
of objects that was not even targeted."

Beers estimates that SEGUE will increase the numbers of VMPs to more
than 20,000, EMPs to several thousand, and UMPs to several hundred;
numbers greater than those found in all previous surveys combined.

Astronomers will analyze the chemical abundance patterns in these stars
to infer the properties of the first stars that formed in the Galaxy and
the nucleosynthetic processes that created the chemical elements.

In addition, notes Beers, SEGUE has the opportunity to turn up
previously undiscovered classes of objects. "By measuring the spectra of
huge numbers of stars, we maximize our chances of finding the rare jewels."

'High velocity' stars

SEGUE scientist Sebastien Lepine of the American Museum of Natural
History focused on measurements of stellar motions, in his presentation
on High Proper Motion Stars in SEGUE and the Local Kinematics of the
Galactic Halo. Lepine explained how he could combine SDSS-II data with
photographic images of the night sky -- taken up to 50 years ago -- to
identify stars that have moved more than a second of arc, or 1/3600th of
a degree, in the last half century.

By combining these measurements of "proper motion" across the sky with
SEGUE's spectroscopic measurements of line-of-sight velocities, Lepine
can build a picture of the 3-dimensional motions of stars in the Galaxy.

"We are particularly interested in a rare breed of stars that move
through the Solar neighborhood at unusually large speeds," Lepine
explained. "These 'high velocity stars' are not part of the disk of our
Galaxy, but rather are members of the Galactic Halo, which contains most
of the oldest stars in our Galaxy.

"By tracing the motions of a few thousand stars from the Galactic Halo,
we will search for 'streams' or 'currents' in the Halo, which will shed
light on the structure, origin, and formation of the Galaxy."

The Open Cluster examination

In the SEGUE Open Cluster Survey (SOCS) presentation, Fermilab scientist
Douglas Tucker, explained how the SDSS-II 2.5m telescope's imaging
camera will scan numerous star clusters within the Milky Way Galaxy.

Because all the stars in any individual star cluster formed at the same
time and from the same cloud of interstellar gas and dust, they lie at
essentially the same distance from the Earth and have similar age and
chemical composition. This internal regularity, with mass being the main
variable that differs from star to star, makes clusters especially
valuable laboratories for studying the formation and evolution of stars
and the global evolution of the Galaxy.

There are currently about 1,600 open clusters known in the Milky Way
Galaxy, Tucker explained, but only about a third have well determined
ages and distances, and slightly more than 100 have well-determined
chemical compositions. Open clusters lie in the Galactic disk, and their
typical ages range from 10 million to one billion years.

"SOCS will provide uniform, precise photometry and chemical abundance
measurements for a huge sample of open clusters," said Tucker. "This
will be a superb data set for studying stellar evolution and the role of
heavy elements in determining the structure of stars."

"The most exciting aspect of SEGUE," notes Newberg, "is the prospect of
building a complete picture of our galaxy. This global picture will
allow us to probe the mechanisms by which the Milky Way and its
constituent stars were created and assembled."

IMAGE CAPTION:
[http://www.sdss.org/news/releases/SE...aseFigure.jpeg (52KB)]
This color image of NGC 2420, a 1 billion year-old open cluster about
10,000 light years from our sun, was observed by the SDSS 2.5m
telescope's imaging camera. The bright red stars near the image border
are nearby stars and the red stars in the center are members of the
cluster. (CREDIT: The SDSS-II Collaboration)
 




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