Quasar is again most distant object
By Steven Schultz
A team of scientists that includes astrophysics
graduate student Xiao-hui Fan has broken the record
for the most distant object observed in the
universe.
Working with data from the Sloan Digital Sky
Survey, they discovered a quasar that registers
higher on the redshift scale than any other object
ever observed. They estimate that the light from
the quasar originated 12 billion light years from
Earth.
That vast distance means that the quasar is a
window into the infancy of the universe. The
scientists are seeing the object as it existed when
the universe was less than a billion years old
compared to its current age, which 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 who works with Associate
Professor of Astrophysics Michael 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," said Strauss. "He has now
discovered more than 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."
Dramatic red color
The Sloan project is a detailed survey of
one-quarter of the sky. Using a 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. When
the survey officially began earlier this month, it
had already produced several important discoveries
(including those by Fan) during the previous year
of calibration and testing.
The images that led to the new quasar discovery
were collected in March from a telescope at Apache
Point, NM. From among thousands of ordinary
celestial objects, Fan selected this particular
object because of its dramatic red color. In
collaboration with scientists from the University
of California at Davis, Berkeley and the Space
Telescope Science Institute, he 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.
One in a million
The image's spectrum also tells 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
they 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 quasars were first discovered in
1963, ever more distant quasars were discovered and
held the record for the most distant objects known
in the universe. Then in the mid-1990s,
ultrasensitive telescopes revealed ordinary
galaxies that were even farther away. The Sloan
survey, which is particularly suited to discovering
high redshift quasars, has returned quasars to
their status as most distant objects known.
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