FOR RELEASE 9:20 a.m. EST January 9, 2002
INFRARED SKY SURVEY YIELDS A BIRD'S-EYE VIEW OF OUR HOME GALAXY

Astronomers sifting through the half-billion stars measured in infrared light by the Two Micron All Sky Survey (2MASS) have produced the first-ever "bird's-eye" view of our Galaxy's complete disk and central bar as traced by stars. These results are being reported today at the 199th meeting of the American Astronomical Society in Washington, D.C. by Michael Skrutskie (University of Virginia, Charlottesville, VA), Ted Reber and Nate Murphy (Amherst College, Amherst, MA), and Martin Weinberg (University of Massachusetts, Amherst, MA).

Over the years, many indirect measures have suggested that our galaxy, the Milky Way, is similar in shape to other flattened, spiral galaxies scattered throughout the Universe. The Earth's vantage point lies within the disk of stars and obscuring dust fills interstellar space. Acquiring information about the Milky Way as a whole is a challenge. Nevertheless, astronomers have been able to estimate the diameter of the Galaxy, establish that the Sun lies 27,000 light years from the center, observe the rotation of the Galaxy, infer the presence of a bar-like structure in the central regions, and identify other galaxies which are probably very similar to the Milky Way.

The work presented today provides a direct view of the structure of the Milky Way as outlined by stars. Using infrared light to peer through the Galaxy's obscuring dust, the astronomers identified 30,000 stars of known intrinsic brightness called "carbon stars" from over 500 million stars detected in the 2MASS survey. Reconstructing the stars' locations in three-dimensional space yields a picture of the Milky Way as seen from a vantage point well above the disk. Evident in the map, and seen directly for the first time, are the cigar-shaped bar toward the center of the Galaxy and the complete outer boundary of the disk of stars. A modern space probe launched from Earth would have to travel for 200 million years to reach a point where it could look back and return an image with similar perspective.

When asked to provide an everyday analogy for this finding Skrutskie suggested, "Imagine you are a member of a marching band standing in formation in a thick fog. The fog is so thick you can only see the band members standing nearest to you. You have no hope of discerning the pattern formed by the entire band. Even if the fog lifts (which we have achieved astronomically by observing infrared light which penetrates our Galaxy's dusty 'fog') it is still quite a challenge to figure out how the band is arranged from your perspective within it. But suppose you have a rangefinder which can reveal how far away each band member stands from you. You can then chart out where everybody is actually standing and see the pattern. Our astronomical 'rangefinders' are called carbon stars. These stars all have approximately the same intrinsic brightness. If you observe how bright one appears to be in your telescope you can estimate its distance, since more distant carbon stars will appear fainter."

A substantial portion of the analysis and data sleuthing for this project was performed by Amherst College undergraduates Ted Reber and Nate Murphy as Summer Research and Senior Thesis projects. Reber notes, "The chance to spend an extended period of time grappling with one question and ultimately emerging with some answers was truly the most exciting and satisfying part of the experience. This work also helped cement my desire to pursue a career in astronomy."

Carbon stars weigh between one and a few times the mass of the sun. Their central furnaces are nearly out of fuel, and carbon is one product of their final nuclear reactions. In these stars, internal motions circulate the carbon produced in the core out to the star's surface where it can be detected in the emitted starlight spectrum --- hence the name "carbon star". These stars are extremely luminous and easily recognizable across the Milky Way in the 2MASS images. Most carbon stars have approximately the same intrinsic brightness. The observed and intrinsic brightnesses of an object are related by its distance, so a carbon star's distance can easily be determined.

Since carbon stars are easily selected from the vast 2MASS database, this work is an early "simple but effective" application of 2MASS results to the problem of deriving the structure of the Milky Way. Future work will attempt to extract the subtle features of the Galaxy's structure using hundreds of millions of 2MASS detections rather than just 30,000. The computational resources to conduct such analyses are staggering but the problem has become manageable as computer technology and research into data mining programs advance in tandem. Weinberg points out that, "By organizing information about the 2MASS star's positions into a 'tree hierarchy' (akin to a mailing address: first locate the country, then the state, city, street, etc.) and using a pile of standard PCs in parallel, our new software tool removes the computational bottleneck found in standard statistical programs."

The Two Micron All Sky Survey (2MASS) project has spent the past four years collecting data which provides a near-infrared view of the entire celestial sphere. Near-infrared light has a wavelength about four times longer the visible light that human eyes can see and penetrates the Milky Way's obscuring dust much more effectively than visible light. Human vision is insensitive to infrared light and such light must be detected by sophisticated electronic cameras cooled to temperatures not far above absolute zero. The University of Massachusetts, led by 2MASS project manager Rae Stiening, managed the fabrication and operation of two dedicated 1.3-meter diameter telescopes -- one at Mt. Hopkins, Arizona the other at Cerro Tololo, Chile. These telescopes scanned the skies every clear night for nearly four years collecting 25 terabytes of imaging data. Dr. Roc Cutri at Caltech's Infrared Processing and Analysis Center in Pasadena, CA led the data processing effort which transformed this vast amount of raw data into astronomically useful images and lists of the infrared brightnesses and positions of more than 500 million stars. Michael Skrutskie is the Principal Investigator for the 2MASS project. Information about the 2MASS project and 2MASS data products are accessible world-wide at http://www.ipac.caltech.edu/2mass

The Two Micron All Sky Survey is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

For more information:

   Dr. Michael Skrutskie (434) 924 4328   
                         skrutskie@virginia.edu
   Dr. Martin Weinberg   (413) 545 3821   
                         weinberg@osprey.astro.umass.edu

The text of this press release and high-resolution color versions of the attached figures are available at http://www.astro.virginia.edu/~mfs4n/milky/

A visible-light (left) vs. 2MASS infrared-light (right) view of the central regions of the Milky Way galaxy graphically illustrating the ability of infrared light to penetrate the obscuring dust. The field-of-view is 10x10 degrees. Photo credit: Howard McCallon and Gene Kopan/2MASS Project The 2MASS view of the entire Milky Way. The results described in this release add "depth" to this picture. Photo credit: John Carpenter and Robert Hurt/2MASS Project
The barred spiral galaxy NGC253 as recorded by 2MASS. This galaxy's bar and disk are reminiscent of the distribution of carbon stars in the Milky Way revealed in this work. This figure shows the primary result of this press release -- the distribution of 30,000 carbon star candidates as seen looking down on the disk of the Milky Way galaxy from above. The green cross marks the position of the Sun. The red cross marks a point 8,500 parsecs (27,000 light years) from the Sun where the center of the galaxy is thought to reside. The two figures plot the same data with different "stretches" to highlight the detail. Evident are the tilted bar at the center of the galaxy and the outer limits of the Milky Way's stellar disk. This technique is not as sensitive to the spiral arms of the galaxy. Only a hint of those structures is apparent in the maps. Individual stars are not plotted as points in this diagram but as shaded lines to account for the uncertainty in their estimated distances. 20,000 parsecs is equivalent to 65,000 light years. The features outside the Milky Way on the left side of the diagram are the carbon stars of the Large and Small Magellanic Clouds.