Astronomers Take Step in Measuring Universe's Expansion
This visible-light image shows the galaxy dubbed UGC 3789, which is 160 million light-years from Earth.
Credit: STScI

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.