Radio telescopes extend astronomy's best 'yardstick'

Jun 08, 2009

Radio astronomers have directly measured the distance to a faraway galaxy, providing a valuable "yardstick" for calibrating large astronomical distances and demonstrating a vital method that could help determine the elusive nature of the mysterious Dark Energy that pervades the Universe.

"We measured a direct, geometric distance to the galaxy, independent of the complications and assumptions inherent in other techniques. 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 ," said James Braatz, of the National Radio Astronomy Observatory (NRAO), who presented the work to the American Astronomical Society's meeting in Pasadena, California.

Braatz and his colleagues used the National Science Foundation's Very Long Baseline Array (VLBA) and Robert C. Byrd Green Bank Telescope (GBT), and the Effelsberg Radio Telescope of the Max Planck Institute for Radioastronomy (MPIfR) in Germany to determine that a galaxy dubbed UGC 3789 is 160 million light-years from Earth. To do this, they precisely measured both the linear and angular size of a disk of material orbiting the galaxy's central black hole. Water molecules in the disk act as masers to amplify, or strengthen, radio waves the way lasers amplify light waves.

The observation is a key element of a major effort to measure the expansion rate of the Universe, known as the Hubble Constant, with greatly improved precision. That effort, cosmologists say, is the best way to narrow down possible explanations for the nature of Dark Energy. "The new measurement is important because it demonstrates a one-step, geometric technique for measuring distances to galaxies far enough to infer the expansion rate of the Universe," said Braatz.

Dark Energy was discovered in 1998 with the observation that the expansion of the Universe is accelerating. It constitutes 70 percent of the matter and energy in the Universe, but its nature remains unknown. Determining its nature is one of the most important problems in astrophysics.

"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).

The work on UGC 3789 follows a landmark measurement done with the VLBA in 1999, in which the distance to the galaxy NGC 4258 -- 23 million light-years -- was directly measured by observing water masers in a disk of material orbiting its central black hole. That measurement allowed refinement of other, indirect distance-measuring techniques using variable stars as "standard candles."

The measurement to UGC 3789 adds a new milepost seven times more distant than NGC 4258, which itself is too close to measure the Hubble Constant directly. The speed at which NGC 4258 is receding from the Milky Way can be influenced by local effects. "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.

Following the achievement with NGC 4258, astronomers used the highly-sensitive GBT to search for other galaxies with similar water-molecule masers in disks orbiting their central black holes. Once candidates were found, astronomers then used the VLBA and the GBT together with the Effelsberg telescope to make images of the disks and measure their detailed rotational structure, needed for the distance measurements. This effort requires multi-year observations of each galaxy. UGC 3789 is the first galaxy in the program to yield such a precise distance.

Team member Cheng-Yu Kuo of the University of Virginia presented an image of the maser disk in NGC 6323, a galaxy even more distant than UGC 3789. This is a step toward using this galaxy to provide another valuable cosmic milepost. "The very high sensitivity of the telescopes allows making such images of galaxies even beyond 300 million light years," said Kuo.

Source: National Radio Astronomy Observatory (news : web)

Explore further: Transiting exoplanet with longest known year

add to favorites email to friend print save as pdf

Related Stories

Radio Telescopes' Sharp Vision Yields Rich Payoffs

Jan 11, 2008

Having the sharpest pictures always is a big advantage, and a sophisticated radio-astronomy technique using continent-wide and even intercontinental arrays of telescopes is yielding extremely valuable scientific ...

Most Distant Water in the Universe Found

Dec 17, 2008

(PhysOrg.com) -- Astronomers have found the most distant water yet seen in the Universe, in a galaxy more than 11 billion light-years from Earth. Previously, the most distant water had been seen in a galaxy ...

Astronomers Measure Slowest Motion Across The Sky

Mar 03, 2005

In the March 4th issue of Science, astronomers report that they have measured the slowest ever motion of a galaxy across the plane of the sky. This distant whirlpool of stars appears to creep along despite its actual speed ...

Hubble Snaps Images of a Pinwheel-Shaped Galaxy

Feb 07, 2006

Looking like a child's pinwheel ready to be set a spinning by a gentle breeze, this dramatic spiral galaxy is one of the latest viewed by NASA's Hubble Space Telescope. Stunning details of the face-on spiral ...

VLBA Changes Picture of Famous Star-Forming Region

Oct 08, 2007

Using the supersharp radio "vision" of the National Science Foundation's Very Long Baseline Array (VLBA), astronomers have made the most precise measurement ever of the distance to a famous star-forming region. ...

Dwarf galaxy has giant surprise

Jan 12, 2005

Huge gas disk may be similar to stuff of early universe An astronomer studying small irregular galaxies has discovered a remarkable feature in one of them that may provide key clues to understanding how galaxies form and ...

Recommended for you

Transiting exoplanet with longest known year

22 hours ago

Astronomers have discovered a transiting exoplanet with the longest known year. Kepler-421b circles its star once every 704 days. In comparison, Mars orbits our Sun once every 780 days. Most of the 1,800-plus ...

Mysterious dance of dwarfs may force a cosmic rethink

Jul 21, 2014

(Phys.org) —The discovery that many small galaxies throughout the universe do not 'swarm' around larger ones like bees do but 'dance' in orderly disc-shaped orbits is a challenge to our understanding of ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

jeffsaunders
not rated yet Jun 08, 2009
Unfortunately, I have read the article and failed to grasp where exactly is the direct measurement.

There is a water molecule powered maser in the center of the galaxy - fine - it amplifies radio signals - fine again. What exactly was measure that tells us how far away the galaxy is? Does anyone know?
lomed
not rated yet Jun 13, 2009
After looking at the NRAO website, I realized that the Physorg article does, in fact, mention the method.
they precisely measured both the linear and angular size of a disk of material orbiting the galaxy's central black hole. Water molecules in the disk act as masers to amplify, or strengthen, radio waves the way lasers amplify light waves."

So, the masers provided sufficiently bright markers for the measurement of the angular size of the disk of material, and through measurements of the motions of the masers (via variations in red-shift, I suppose), a good approximation could be made for the linear dimensions of the disk. Given the angular and linear dimensions of an object, it is a simple matter of trigonometry/proportions to obtain the distance of the object.