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April 13, 2000

Scientists Discover Most Distant Object Ever Observed

PRINCETON, N.J. -- A team of scientists that includes Princeton astrophysics graduate student Xiaohui Fan has broken the record for the most distant object ever observed in the universe.

Working with data from the Sloan Digital Sky Survey, the scientists discovered a quasar that registers higher on scale known as redshift than any other object ever observed.

Translating the redshift scale into an actual distance is inexact because it depends on difficult and uncertain measurements, such as the rate of expansion of the universe. Using currently estimated values, however, the newly discovered quasar is roughly 12 billion light years away from Earth, said Princeton Associate Professor of Astrophysics Michael Strauss.

That vast distance also means that the quasar is a window into the infancy of the universe. Because it took 12 billion years for the quasar's light to reach Earth, the scientists were seeing the object as it existed when the universe was less than a billion years old compared to its current age, which (again, using current estimates) is thought to be about 13 billion years.

The quasar edged out the previous record holder for most distant object, a galaxy with a slightly lower redshift.

The discovery exemplifies not only the power of the Sloan survey, but the skill of scientists such as Fan in interpreting the avalanche of data streaming from the ambitious project. Fan, a fourth year graduate student under Strauss, was this year's recipient of the Porter Ogden Jacobus Fellowship, Princeton's highest honor for a graduate student.

"Xiaohui was honored with the Jacobus Fellowship for his previous work finding high-redshift quasars with the Sloan survey. He now has discovered over 100 high-redshift quasars, of which this discovery was the most dramatic, and has also found a whole host of other interesting objects, including the coolest stars known," said Strauss.

The Sloan project is a detailed survey of one-quarter of the sky. With an imaging camera developed by Princeton astrophysicist James Gunn, it collects data on objects that are thousands of times dimmer than could be detected by previous surveys. The survey officially began earlier this month, but had already produced several important discoveries (including those by Fan) during the previous year of calibration and testing.

The images that led to the discovery were collected from a telescope at Apache Point, New Mexico in March. From among thousands of ordinary celestial objects, Fan selected this particular object for its dramatic red color. In collaboration with scientists from the University of California at Davis, UC Berkeley and the Space Telescope Science Institute, Fan used the 10-meter Keck telescope on the summit of Hawaii's dormant Mauna Kea volcano to confirm that it was a high-redshift quasar.

That confirmation required measurement of its spectrum, splitting its light up into its component colors. This spectrographic data is critical not only in identifying objects but in telling how far away they are (by measuring redshift).

As the universe expands, light emitted from distant objects becomes stretched, making its wavelengths longer and therefore shifting the light to the red end of the spectrum. The greater the time and distance the light has traveled, the more pronounced the redshift.

The image's spectral features also tell scientists what it is. The quasar was identified as such because of strong emissions from hydrogen, the most abundant element in the cosmos. Quasars are objects that resemble stars in that they appear point-like, but are actually powerful emissions of light that result from matter pouring into an especially massive black hole at the center of a galaxy. Normal stars are far more common than quasars, so "picking out the quasars is like picking a needle out of a haystack, and the high-redshift ones are particularly rare - one in a million is not an exaggeration," said Strauss.

Soon after their discovery in 1963, ever more distant quasars were discovered; they thus held the record for the most distant objects known in the universe. Then in the mid 1990s, ultrasensitive telescopes revealed ordinary galaxies that were farther away yet. The Sloan survey, which is particularly suited to discovering high redshift quasars, has returned quasars to their status as most distant objects known.


For images and a more detailed news release, please visit the Sloan Digital Sky Survey website at or by calling the Princeton University Office of Communications at (609) 258-3601.