PASADENA, CALIF. — Astronomers have directly
measured the distance to a faraway galaxy, providing them with a yardstick that
could help determine just how fast the universe around us is expanding.
"Measuring
precise distances is one of the oldest problems in astronomy, and applying a
relatively new radio astronomy technique to this old problem is vital to
solving one of the greatest challenges of 21st century astrophysics," said
team member Mark Reid of the Harvard-Smithsonian Center for Astrophysics (CfA).
Using the
Very Long Baseline Array (VLBA) in New Mexico, the Robert C. Byrd Green Bank
Telescope in West Virginia and the Effelsberg Radio Telescope in Germany, the astronomers determined that the galaxy UGC 3789 is 160 million light-years
from Earth.
The work
was presented here today at the 214th meeting of the American Astronomical
Society.
With the
radio signals observed by the telescopes, they precisely measured the linear
and angular size of a disk of material orbiting the galaxy's central black hole.
Water molecules in that disk act as so-called
masers to amplify radio waves (in the same way, lasers amplify light
waves).
"We
measured a direct, geometric distance to the galaxy, independent of the
complications and assumptions inherent in other techniques," said team
member James Braatz of the National Radio Astronomy Observatory (NRAO).
"The measurement highlights a valuable method that can be used to
determine the local expansion rate of the universe, which is essential in our
quest to find the nature
of dark energy."
Dark energy
is an unexplained force that is pulling galaxies away from each other at an
accelerated pace and is thought to make up 70 percent of the mass and energy in
the universe. The rate of that pull is called the Hubble Constant.
Right now
the Hubble Constant is known to within about 10 percent of its suspected value.
The most widely accepted value is 72 kilometers per second per megaparsec,
obtained by the Hubble Space Telescope. (One megaparsec is about one million
parsecs, which are equal to about 3.26 light years.)
The
distance to UGC 3789 isn't the first to be measured directly with this
technique – in 1999, measurements done with the VLBA used water masers to
directly measure the distance to NGC 4258, 23 million light-years away. But
that distance is still too close to accurately measure the Hubble Constant
because the movement of NGC 4258 away from the Milky Way could be influenced by
local effects.
The
distance to UGC 3789 extends
the milepost seven times more distant.
"UGC
3789 is far enough that the speed at which it is moving away from the Milky Way
is more indicative of the expansion of the universe," said team member
Elizabeth Humphreys of the CfA.
At least 10
more galaxies as far away as UGC 3789 will be needed to refine the Hubble
Constant to within 3 percent, Braatz said.
"The
goal is an ambitious one. We don't know yet whether you can achieve that,"
he said.
His team
has already surveyed 2,000 galaxies and found 120 with water masers, seven of
which occur in a disk as in UGC 3789. "Of those seven, we might get good
distances for three or four," he said.
Astronomers
have their eyes set on an even more distant galaxy, NGC 6323. The maser disk of
this galaxy has been image by Cheng-Yu Kuo of the University of Virginia.
"The
very high sensitivity of the telescopes allows making such images of galaxies
even beyond 300 million light-years," Kuo said.
But
astronomers won't have to settle for a 3 percent value of the Hubble Constant
forever, Braatz said. "We could eventually do better than that."
Bigger telescopes, such as the Square Kilometer Array, currently in the works,
could refine the value even more.
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