January 11, 2006
The Sloan Digital Sky Survey turns its eye on the galaxy
A closer look at the Milky Way
WASHINGTON — 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 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 (kpc) 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."